It doesn't work for everyone, but I've enjoyed using #anki in the past. It seems #AnkiWeb has some user-contributed #Gaelic lessons: https://ankiweb.net/shared/info/1357645930

On Android, I use #AnkiDroid from #FDroid

Perhaps #AnkiDroid could be useful here? There are free sets of cards for an incredible amount of topics and languages. #Anki is about spaced repetition for learning stuff.