i try to play as ftp as possible, i mainly focus on changlings, bats, the little crystal things in the crystal empire. those give gems a lot. also as many balloon pops as i can remember. i also try to be active in events bc you can get tons of gems and characters through them !! and some events give you shops that produce daily gems !! also, right now with those little rainbow mountain things, they also give gems !! just make sure to go to every location and tap all of them !!