Another way to earn gems is to play selected levels. Many of the levels in the mountain and the glacier have detailed instructions and are easier than Clash of Clones. In plus you have many topics in the forum where almost all the possible questions are already answered. You can access the mountain and the glacier directly:
and then type https://codecombat.com/play/level/name-of-the-level
( mouse over the level to see the name)
List of all levels in codecombat: Levels List w/ Links
I have free accounts for python, java-script and coffee-script and didn’t need gems at any single moment. If you are a subscriber ( as my son ) you really need gems for heroes, but I strongly suggest first to pass all levels with a simple hero to learn the programing concepts.
I think this area in the mountain is relatively easy:
