You don't have to gem, although if you have some, it does make life easier.
This is all you have to do: roll an army of decent strength. It doesn't have to be crazy expensive. You can cook a pretty decent army in about an hour. Go attack, and take some loot. It's important to attack villages where you know you can win. You just queue up an army, and wait for your phone to give you a notification that it's done. That will take less than 1 minute.
Come back, find a base, and liberate as much loot as possible. 5-10 minutes. Rinse and repeat. Always attack through your shield. You can get in 2-3 attacks while the shield is up. When it goes down, assuming your attacks have been successful, you should have liberated a lot more loot than you lost.
Hint: the higher your trophy count, the better your loot bonus. That loot bonus goes a long way.
I can work, cook dinner, etc while waiting for my army to produce. The vast majority of the time I'm doing something else.