hi my 95 sierra 1500 with 5.7litre has all forward gears but no reverse
Try moving the shifter very very slowly from neutral to reverse to park. If nothing happens then it sounds like something inside the trans is bad, either electronic or mechanical. Either way it will have to go to a trans shop to find out which one it is.
Search GMC Sierra 1500 Questions
GMC Sierra 1500 Experts