That is what that means, yes. However, you can have several builds that have an "a" designation without ever getting a GM. It just means that a) The build compiled successfully, and b) They were satisfied with internal testing that nothing was super broken. A build that compiles successfully and passes internal QA doesn't mean more than that.
The best tool that people outside of the company have to gauge how far along Apple is is the size of the OTA download. It's a delta update, so the bigger the OTA, the more changes. The smaller the OTA, fewer changes. As development on a release winds down, you'll usually see smaller OTAs since they aren't committing as many changes.
That said, even the OTA download size isn't necessarily a great predictor. We've had betas before where we get several <75MB OTAs before GM, and we've had betas where we're still seeing >100MB OTAs before GM.