When I start to think about running a science fiction game, I tend to think fairly small. I want the Basic Set, and I’m likely to grab Ultra-Tech if only for a more robust equipment list to select from. If starships are likely to be more than plot points, I’ll likely bring in at least the first few Spaceships PDFs. If I’m not using an established setting, I’ll break out Space for the star system design tools. It’s not a huge list by any means, and honestly everything aside from the Basic Set is gravy.
Fantasy on the other hand brings out the detail fetishist in me in a big way, and I quickly escalate to at least a dozen books. Low-Tech leads to Instant Armor and Loadouts; Magic begets Thaumatology which leads inevitably to Magical Styles and/or Ritual Path Magic. I rarely even bother to include Fantasy in my list, but Powers, Martial Arts, Technical Grappling and Divine Favor are often added to the pot. And yet, again, everything outside of the Basic Set (except maybe Magic – the default magic system is a little light as presented without it) is gravy.
Whenever I find myself thinking that the GURPS ecosystem is getting too large (look at Technical Grappling if you want to see an example of exponential complexity – so many what-if rules based on a dozen or a hundred prior supplements), it is nice to reflect on just how little of the system you really need – the Basic Set alone – to play the game effectively.