Yes.

Also, more often than not, OS X GM is the version that's released to the public.

Even if they do update GM versions, it's still a delta update between the final GM and the subsequent release (10.11.1 in this case)

I believe in the past, it was not the case. I remember once I downloaded the GM and a day later there was a new one. I went to software update and there were no pending changes. I had to wipe/start over.

With the 10.9 first GM you couldn't update to the second GM, but updating to 10.9.1 worked fine.

How can you be certain that the result was identical?

Because science. Also deltas.

It's because "new one" wasn't actually new, only difference was in naming.