I think it’s better to think of all the add-ons and supplements as GM inspiration, rather than hard and fast rules. Most everything in GURPS is set up to arrive at a skill roll or attribute modifier; so even if you don’t remember a particular rule for a particular edge case, you can generally eyeball it and come up with a modifier pretty close to what’s in the books. The books give a lot of guidance on how to reach that modifier, though; and give you enough information to feel comfortable coming up with your own modifiers outside of what they outline. I feel like that’s a lot of what GURPS brings to the table - a simple system, with an internally consistent set of guides about how easy or hard a given action might be.