D&D Third-Party Material and GURPS
You probably know that D&D 3.0 came out with the OGL license that allowed third-party publishers to publish books for it, and the same thing happened with D&D 3.5 and Pathfinder (and D&D 5e too, I think, but I'm not really sure, I'm not intimately familiar with that iteration). Some see it as a boon, some see it as a bane. I have a large collection of various 3pp books for D&D and Pathfinder, and it's probably no surprise to anyone, but I've almost never used them. Why? Because most of it is absolute garbage in the mechanical sense - it was either underpowered, overpowered, broken, or simply unneccessary due to the system's specifics. However, some of the concepts I've found in those books are fascinating. Some introduce something new to the game, some expand certain niches or bring interesting details into the world that make it feel much more special and alive. At least when it comes to D&D, because I noticed a sharp decline in quality of 3pp material once Pathfinder came out. There's much more 3pp Pathfinder stuff, but it's so, so much worse (with some rare gems, of course). Oftentimes I open a pdf and become puzzled with the fact that people have had the audacity to sell that.
What's so great about GURPS is that you can take any of those fascinating concepts and adapt it to GURPS, preserving the flavor and expanding your game world, and it will make sense mechanically too. For example, if I were playing D&D and opened The Quintessential Druid by Mongoose Publishing, I'd groan and say that I would never integrate that into the game. Now, I open it and see so much potential! The entire Quintessential Series of books is a treasure trove of things to enrich your GURPS experience. The same publisher has other books series, such as the Encyclopaedia Arcane/Divine/Psionica. Then there's other big publishers, such as Green Ronin, Sword & Sorcery, Dreamscarred Press, and many smaller ones. You have so much material on your hands! This is something I love about GURPS - you're not limited to just GURPS books, you can use anything.