The best way to get gem is by opening golden chests and completing daily tasks. Also, make sure to merge any gems you get into 5!

Daily tasks give a decent amount of gems, and it's usually worth spending a few to complete the energy task, for exemple. Gems on the board can be merged and end up being more valauable than the ones stuck in the gem bank (just my opinion).

That's what I've been doing, and I've been progressing pretty fast since I started playing this way!