They have several setting/adventure books, a lot of IPs choose to my heart: Star Trek, Discworld, Girl Genius. You can get their Infinite Worlds book which is kinda the default setting. It's a multiverse thing so technically your homebrew is canon, but they lay out a bunch of What-if settings like "What if the Nazis won?" or "What if Rome never fell?".

There's also Dungeon Fantasy, which is a Powered by GURPS module which gives you pre-built character templates and enemy stat blocks and adventures and stuff. If you're used to D&D, it's a fairly gentle transition with familiar themes.

The thing about the challenges and side interactions though, is that once you get a feel for setting difficulty modifiers, you can import that stuff from anywhere. The best part of GURPS is running homebrew. Its strength is that you can write any scenario you want, with the confidence that you can make it work mechanically somehow.

The people on the Discord are generally pretty helpful if you need help, although just about anything you want to do has already been theory crafted on the Steve Jackson Games forum.