Originally Posted by
ScottCommutes
Just speaking from my own experience. I have three old cast-off 90's style mountain bikes with 26" mountain wheels (ISO 559) that my son, my wife, and myself ride - a Schwinn, a Mongoose, and a Diamondback. None of the rear wheels on any of them are interchangeable because they have 126mm, 130mm, and 135mm rears. I can't find brand new rear wheels for them at all anymore except for the 135. Two of them are also bolt on axles.
There was a short period of time in which MTBs transitioned from older road dropout spacing to the more modern 135mm, but mostly on cheaper bikes from the late 80s/very early 90s that used freewheels. At this point you won't find a replacement for the two narrower spacings, I can recall having to stretch a couple of frames slightly when we could no longer buy MTB wheels in narrower spacing and had to make more modern wheels fit. Assuming 135 on a 90s GT MTB would be considered a safe bet.