Since getting gems can take a while to rack up, I think an option to before a match gamble them on a % basis to gain more, so say you can earn 100 gems a match, if you gamble 100 gems you get 200% for that match, say you got 360 gems you have to gamble 360 for 100% more, 180 gems would net you 50% more, etc etc.
Just gamble with Cinder - he likes to make a wager or 2…
There is a mission with TJ I have come across that allows you to wager up to 250 for the match. Granted it’s not too frequent, but there is that option.
This is what I started doing. Select easy mode, end all days until first Omen shows up. Beat omen, then pass days until Gargos. Beat Gargos. You will only have 2 fights and get between 1000 and 1200 in about 5 min. Repeat. 