I vaguely remember trying some demo.
Let's see... I do have in my PlayStation 1 demo discs "Gex 3D" (apparently the second game Enter the Gecko) and Gex 3: Deep Cover Gecko. Not going to dig through whether I have a PC demo somewhere.
Judging by my old notes (you have no idea what kind of archives I have for everything...) I liked the third much better than the second.
But platformers, especially the early 3D platformers of late 90's, are really not my thing.