OpenBoard is a great FOSS keyboard on F-Droid.
Discussion
Floris and Simple are good choices as well.
Nice. Trying Floris right now. Using it to reply to you. Looks the most similar to iOS keyboad on Android I think. Very slick wow.
I use it exclusively for the gesture typing. I've gotten so used to it over the years that I type much quicker. Sometimes it has issues and errors but is the only FOSS option I have found that has it.