I haven't really found a particularly good alternative. I used to use AnySoftKeyboard, but stopped after I found that it was the cause of some performance issues I was having that would occasionally cause characters to be entered out-of-order on a slower Android device.
The lack of decent open source onscreen keyboards was actually one of my larger surprises when I hit Android. I thought that there'd be keyboards with user-specified macro keys, keys that could be dragged into a new layout or added and removed, etc, but no.
I have been using OpenBoard recently and it feels exactly like GBoard, except I have an issue where using backspace/deleting characters also randomly removes spaces from words I've already typed. So words will start gettingmashedtogetherlikethis. Unfortunately it seems unmaintained so I guess I will just have to move back to GBoard for now.
Well, there is Florisboard but it hasn't received an update for about 10 months. (Edit: found out its still developing but I think its sloooooowww)
Openboard is ok.
AnySoftKeyboard is ok.
Simple Keyboard looks too simple.
Eh, i use florisboard. it doesnt exactly suit my needs, but it gets the job done.
GBoard is only fine if you live in US and speak no other languages :)
It doesn't support automatic space addition after punctuation outside of English, doesn't support combined keyboard layouts (writing thos from a combined de/en layout on SwiftKey).
With their recent updates im just glad they finally added split keyboard functionality to the rest of the android tablets. Im actually texting quickly on a 12 inch samsung tablet
The samsung keyboard sucks when using the split keyboard function.
If you go into a input field on Android and you type a key, it doesn't send that key. It sends "key chords" through a separate event system which are an absolute bitch to deal with.