Gem is rare so you shouldn't spend it on stamina refill. Since you are tired of doing gacha you can use fg to buy manyus and ampys in shop, it's also an efficient way of using gems (Dont take this opinion serious ) . Otherwise, doing map 12-5 or whatever map in stage 12th campaign to get the bonus stage, i have done it before but mostly for leveling up. Also on Manyus event day, log in every 4 to 5 hours to maximize gathering.