My coach in college taught me the "below the throat" idea, where if you have any symptoms below your throat, you should hold off from training. Any muscle aches, back pain, lung tightness, rashes, etc. If it's really and only a head cold I'd bring up these two points:
- Don't infect other people. The people going to that gym will be close to you, using the same gear as you, and don't want to be sick. If you're contagious, stay home.
- Some colds go on for weeks, so it's a little ridiculous to not train until you are 100%.
I think you might want to shoot for something like 70% of your previous work load. Enough that you're in maintenance mode, but not enough that you're overly taxing your body and causing adaptation. As you feel better, throttle back up to where you were at.