Having all those options does require that the GM select all those optional rules. I understand that Sean Punch, GURPS Line Editor extraordinaire, selects optional rules on the fly, using really detailed rules for individual duels but then dropping all that detail and going to broad strokes for big fights. That's amazing, if true, but I prefer to have one set of optional rules selected in advance so that all the players are on the same page. These are the optional rules I selected, and the house rules I'm using, and now I'm going to explain why I chose what I did.
What Books Are We Using?One of the ways to decide on optional rules is to decide what rule and setting books are going to be included, and which aren't. Even though I'm implicitly allowing psionic powers in the game, I'm not using GURPS Psionic Powers, because I personally don't like that book, especially it's highly optional rules for power stunts that my players tend to treat as gospel. On the other hand, I really like Imbuements, so that was definitely in.
With only a few exceptions, I'm using most of the "core" and "expanded core" books from GURPS: Characters and Campaigns, of course, but also Magic, Martial Arts, and Powers. I'm also using almost all of the Power-Ups series. Powers: Divine Favor is a flavorful expansion of Powers, so I'm including it. Thaumatology is a big book of optional rules, most of which I didn't want to use because I was already house-ruling magic, so it's available as a reference for a couple of things but it's generally not in use.
Possibly the most difficult set of optional rules to decide upon are the grappling rules. Campaigns has a perfectly functional, if rather binary in outcome, set of grappling rules, and there's a couple of variants in Pyramid, and then there's the big hammer in Technical Grappling (TG). Since +Douglas Cole is the author of TG and also a player, it means I have the author on hand to explain any confusing bits. That doesn't quite make the inclusion of TG a no-brainer, but it lowered my resistance to adding a more complicated set of rules for a relatively rare combat action. Doug did propose using a draft of his simplified TG rules, but I got about 8 pages into a 50 page document before realizing that they weren't that simplified and nixed that idea.
Lots of Options, Pick a FewAlthough I'm using a lot of the books, at least in theory, I'm not using a lot of the optional rules. Generally, official option rules tweak a game in one of two directions: gritty realism or over-the-top cinematism. I didn't want a gritty game, because that's incongruous with dungeon-crawling, but I didn't want a crazy cinematic game either. Basically, I wanted an action movie, but something along the lines of the first Die Hard, not Die Hard With a Vengeance.
For the most part, I chose optional rules that are clarifications and logical expansions of the Basic set rules: Committed and Defensive Attack or the new Extra Effort options. The only new Extra Effort option I don't allow Heroic Charge in my games, because my experience is that it makes a mockery of defensive positioning: Heroic Chargers run behind enemy lines and strike people from behind, and what should be a ground-based melee looks more like an aircraft dogfight.
Pyramid OptionsPyramid is the monthly GURPS magazine, and contains a huge collection of optional rules. I selected a very small subset of them, initially sticking with the "logical extensions of the standard rules" theme: so All-Out (Strong) can be used for thrown weapons and the like. The one exception was another one of Doug's articles, "Delayed Gratification", which adds a new attack option that is something of a combination of a feint and a deceptive attack. Originally, I didn't include it because I've almost never seen it used in play by anyone other than Doug, but he asked for it specifically and I didn't mind adding the option for him. I expect it will almost never come up, since the PCs are using guns a lot more than fists or blades.
Rejected Optional RulesI did propose a couple of other rules from Pyramid that got rejected. David Pulver has a suggestion to halve the damage of long-arms and give them an armor divisor, giving them the same penetration against armor but reducing their lethality. We were going to use that, but I finally decided it helped the bad guys (who would be the target of longarms) more than the PCs (who be firing the longarms), and that it made using GCA complicated, since the firearm damage on everyone's sheets would be wrong. It was too much complication for too little gain.
Moving Beyond the Optional RulesEven with all the official optional rules, there comes a time when a GURPS GM wants to tweak the game to better match his tastes. I'm not immune to that impulse, though this time through I tried to rein it in as much as possible.
The first change I made was halving the cost of ST. That was a change I first tried in Mecha Against the Giants, and I really think it's helpful for high-tech games. First, it recognizes that ST isn't worth as much in the age of gunpowder. Second, it discourages people from selling down their ST: ST 9 only gives 5 more points, and the reduced HP and lifting capability can really effect people so the trade-off is less worth it. Finally, it actually encourages people to buy up their ST a bit, because the extra HP and increased lifting capability are worth it. If ST had been 10 points per level, I expect everyone would have had ST 9, 10, or 11, but the actual PCs are grouped around 12 with a little more variation.
I came up with several perks, mostly from comments by +Nathan Joy. There's been a constant struggle in house rules in our online games, between the people like him that think Fast-Draw should be a weapon technique, and people like me who think it should be a skill or something. This time we found a compromise: Fast-Draw Training is a perk that lets you roll against weapon skill to fast-draw it. People still put a point in it, but it's one less skill. This idea really came into it's own by adding a similar perk for Forced Entry that let people skilled with weapons get bonus damage for attacking stationary objects. This saves a couple of points for focused warriors, but also lets non-combatants (such as firefighters) learn the Forced Entry skill. It's an elegant solution.
Another concept I borrowed from Nate is weapon skill categories. GURPS weapon skills are possibly overly detailed, and there's a long standing problem that it's stupidly expensive to have a samurai who can use a katana in one-handed and two-handed styles. Weapon skill categories are a skill category more difficult than the individual weapon skills, but cover a broader category: so Sword Weapons instead of Broadsword and Two-Handed Sword. It basically makes individual weapon skills a specialization of the category skills. Again, it's an elegant solution that allows for an affordable, broadly trained character or a slightly cheaper specialist.
Another skill related change was halving the cost of techniques. I've never liked technique pricing in 4th edition: improving a skill overall by 1 level costs 4 points, and buying two techniques that each improve some subset of that skill by 1 level also costs 4 points, so people generally only buy one or two techniques at most. Halving the cost of techniques means that it's worthwhile to invest it three or four techniques, which will hopefully give a little more variation and diversity in the PCs' abilities.
I'm also using a bunch of house rules from No School Grognard: Better Fantasy Armor, College Ritual Book Magic, SSR Rapid Fire, and revised knockback. I've already written essays about what I didn't like about the standard GURPS rules for each of those house rules, so no need to go into more detail there.
One last change was some different Fright Checks rules. The standard GURPS Fright check rules are odd: a scared character usually freezes up (or throws up), but almost never runs away. Freezing up, instead of running, is especially annoying in online play, because it can mean an hour or more where a player can't contribute at all to the game but still has to stick around. Running away, even if it's a forced action, is something the player has some control over (even if it's only picking the direction that they run) and has the added bonus of potentially triggering new events. The new fright check rules encourage running away by giving characters substantial penalties to all die rolls, including defenses, for failing fright checks. A standing -5 penalty is fairly crippling, and it encourages PCs to retreat from the fight rather than whiff and open themselves up to damage. I like rules that emergently create behavior instead of prescribing it, so this rule is pretty good.