I'd guess "huge investment" in this case is relative. The maintainer is not spending a ton of time building features for the CLA tool since it's mostly "done" and so investing more time to build support for Gitlab would require many more hours of development than they're probably dedicating right now.

And i can imagine that maybe they didn't abstract communication with Github enough and would need to refactor the system to handle that as well.

Generally, i think it's not totally reasonable to expect them to do more free work to support use cases that the maintainer does not need. Since it's open source, we're all welcome to contribute back.