I would definitely go with OpenBoard or FlorisBoard which are FOSS Software. FlorisBoard is still early on so it can become buggy after a lott of use at once too me, but they are constantly working on it. Its the closest open source keyboard iv seen come to GBoard so far... OpenBoard is a great open source one as well, I just wish it had a clipboard tray. It's a pretty simple board which I like, never have problems with it.
Im into the whole Privacy thing, have been before phones lol so im not big on apps from the Play store loaded with Trackers while Google or even Microsoft owning Swiftkey now, are logging the key strokes.. Ha, keyboards don't need an Internet Permission, and GIFS are not a good enough one...