To say some service is "Free" (for now) means you're paying something that isn't disclosed. Even if you're paying in time as beta-tester, you're still paying. And you're still paying in data.
Whereas, GitLab on-prem install is largely under MIT license, which is widely considered to be a very permissive license. I could see the FTC coming to similar agreement with that statement.
Personally I prefer demo, because it's a demonstration of what you can expect.
Freeware could be a good term, but wouldn't it still have the same nothing-is-free issue that GP brought up calling it "false and deceptive advertising"? The term certainly doesn't connote the "why" behind the offering