[Kromm]

I will point out that GURPS Thaumatology: Magical Styles gives extremely detailed guidelines for altering spell prerequisites and moving spells between colleges, along with examples. The goal is to enable the GM to restrict wizards to logically related spell lists without falling back on the rather blunt and arbitrary colleges of GURPS Magic. If you want a guild of Gate wizards or Meta-Spells magicians, that's easily arranged.

Obviously, if you're inclined to see GM pronouncements as "blunt and arbitrary," then you won't like Magical Styles any better . . . but most gaming groups are capable of accepting that fine-tuning content to the game world isn't an issue of arbitrariness, but rather one of consistency and world-building. Like everything else in GURPS, Magic is a tool kit that's intended to be pared down to fit the GM's vision of the campaign, not a list of things to which everyone who buys Magery is automatically entitled. It outright states that not all spells will exist in all settings, and that some may require steep Unusual Backgrounds even if they are allowed. You only get to claim limitations on Magery when you voluntarily limit yourself to a subset of the spells in the campaign.