FormerlyHemlock
Hero
It's mildly unfortunate that the GURPS community tends not to do this--GMs should select which advantages and skills are available as part of setting development, adjust point costs to shape the genre ("in this setting, Magery costs 25 points per level instead of 10" to make magic more rare; "ST is half-price because guns are available"), etc.One of the necessary tasks when using a generic system for a specific setting with specific themes is to adjust the game to fit the setting. Otherwise you have a game who's mechanics do not reflect the setting properly, and that includes incentives and limits.
In practice the tendency (not universal but common) is to give players access to all the GM tools, using the point costs given for the default metacampaign in GURPS Basic, or whatever point costs are in whichever supplement the advantage was first introduced in. Then GMs use an ad hoc veto or Unusual Background to increase (never decrease) point costs to taste.
IMO it's kind of a mess, and I wish the community were more oriented around using GURPS to construct games instead of trying to play with GURPS directly. Even just handing a new player a list of 20 advantages he's allowed to buy is an improvement over having them pore over an (alphabetical!) list of dozens of advantages like Altered Time Rate and 360-degree Vision that are mostly irrelevant to them and illegal to buy.