I've never played a game of Quake in my entire life. I (and most of the rest of the Windows 95 team) played DOOM, but after a while, first-person-shooter games started giving me a headache. By the time Quake came out, I had already abandoned playing FPS games.
I don't remember what it was that I did specifically, but it was along the lines of helping them with various technical issues related to running under Windows. At the time, I was a kernel developer, and the advice I gave was almost certainly related to memory management and swapping.
Sorry it wasn't anything exciting.