Squeaky Floors - What to do?

The gypcrete is definitely the subfloor. Now, what is happening? I've never heard of the kind of problems you're having with this particular product. I'm really not qualified, I don't know much about gypcrete products. Maybe they didn't follow the installation instructions when they laid it? That's really the only possibility if you are having problems. I guess that there could be some problem with the structural members, but even then the creaking caused by deflection wouldn't be noticeable enough for you to complain about it. Call the builder.