I just wanted to mention that FlorisBoard is very similar to Gboard in appearance and overall UX. The one critical feature it lacks is word suggestions (predictive text), but that is expected to arrive with the 0.40 release (currently at v0.3.16).
To answer your question though, one of the things that Gboard lacks, from a security viewpoint, is transparency. It is proprietary software and so the source code is unavailable for review. OpenBoard, AnySoftKeyboard and FlorisBoard are all open source projects, providing transparency with respect to privacy and security.
That said, I too use Gboard as it provides me with the best experience, but I do look forward to FlorisBoard v0.40.