I have implemented a spelling checker algorithm and I wondered if I could add it to Google board(Gboard)(use it instead of google spelling checker) or there's no other option than creating my own keyboard and add my spelling checker algorithm to it ?
There is not a way to add custom spell checkers to Gboard. You're correct - you'd need to create your own keyboard (unless there are other keyboards that do allow this somehow, but I'm not aware of any). You could probably use an open-source keyboard as a base if you wanted to do that.