Ehhh, once it clicks, GURPS boils down to 3d6 ≤ skill level + modifiers. All the "game settings" are just tools to accurately map the character and world concepts into appropriately priced skill levels and modifiers. The more comfortable the GM gets with pricing, the more they can just wing it and go with a modifier that feels right.

You don't "swap" rules, you use the same rules for everything. All the rulebooks are just guides for keeping modifiers balanced, although the canonical GURPS universe is predominantly a setting-hopping/genre-changing monstrosity so that might be of interest to you.