Right as it is now you can only get gems from either leveling up or paying REAL LIFE money.
You need gems to buy skins and battlepasses. You need to get to level 25 without spending a SINGEL Gem to be able to get the battlepass which in my opinion is way to hard, so im suggesting that you would be able to earn Gems by either some kind of Mission ( example ) getting 40 kills in 1 hour or suriving a round for 5minutes and each mission gave you maybe 2 Gems. Or another suggestion is that you watch ads to get Gems like 1min vid for 1-2 gems. But to make it so you wouldent get to many Gems you could only do like 3 mission a day or only watch 3 ads a day.
Also maybe add more stuff to do with gems like either more skins or bring back uppgradeable powerups, trails to bullets / diffrent color to bullets, trails from player or different aircrafts sepparate from the skin of the line and if you would add that it would add some more customization to the game.
Anyways hope that you would consider my suggestion as i dont think im the only one feeling like this and i think that it would bring some more life to the game.