I started a thread on the official SJ Games GURPS forum about this question, and present here my collation of the responses:
The game assumes that there is a GM, and that their rulings will be sensible, at least for their desired game. A GM does have to establish what kind of game they want to run, and it is assumed that the overall game objective is fun for the players and GM. Enabling adversarial play is not a priority, nor is ensuring that play will comply with a specific authorial vision.
GURPS is a toolkit, not a game-ready-to-play.
It is not expected that everything from all published GURPS 4e supplements, or even everything in the core rules, will be available to characters in any given single campaign. Some rules are mutually exclusive, on the grounds of different assumptions about genre and play style. For example, the gritty-realism martial arts or shooting rules likely have no place in an anime campaign set in a Japanese high-school.
"Use the rules you need for your game, but no more."
It is quite normal for campaigns to have some house rules, or conventions about which optional rules will be used. Changing these during a campaign is best done with player agreement, if at all. Do not expect different GM's campaigns, or even two campaigns run by the same GM, to be completely compatible.
GURPS is a point-buy system
If you want something, you should buy it, rather than trying to contort rules into giving it to you for free. Disadvantages are only worth points if they will cause you problems, likewise limitations on advantages and other traits. Buy the effect, not the description: the description and special effects can be defined later.
The "ABC" principle
Abilities should be
- Accurate, to your vision
- Basic, in that it uses the least convoluted of the possible ways of buying it
- Cheap in that if there are several viable ways of doing something, use the one that costs the fewest points is a good guideline but not an iron-clad rule.
Character design involves social mechanics and personality-defining traits, as well as combat abilities. The game mechanics don't ensure that every character has something to contribute to every problem, that's a matter for character design.
Play style concepts
The default play style assumed in the rules writing is "realistic cinematic." Characters need to make plans, assemble resources, and use good tactics. Things that are difficult in reality are difficult in the game, and it is assumed that players, guided by the GM, will make reasonable efforts to find bonuses for equipment, taking time, and so on to improve their odds of success. Characters should not assume that everything they might want to do will be possible for them; finding the easier way to cope with a problem is sensible.
The source material for the game is quite varied. Because the game is intended to be generic and universal, it concentrates on the basic foundations of storytelling, rather than current fashions. You're as likely to see The Scarlet Pimpernel referenced as Firefly or Mass Effect.
The rules are written in informal language, and are not intended to support legalistic interpretation, especially across multiple books. They say what they mean to say, and if they do not say something, that should be viewed as intentional. They are not written to be proof against exploits and rules holes: dealing with those is part of the GM's job.
Because there are many optional rules, there may be several RAW answers to a question. Deciding how things work is the GM's job, although suggestions from players should be considered. More recently published rules are intended to take priority over older ones. Specific rules override general ones. Generalising specific rules should be done with caution, if at all.
An appeal to reality, or at least Occam's Razor for things that are not real, is generally worth considering. Using rules interpretations that make sense and are fun is more important than sticking to the letter of the rules.