There are very few methods to get free gems in clash of clans if you are not using a hack. First of all, you can clear all the obstacles such as trees, stones, bushes, trunks, and gem boxes. The second option is to complete the achievements that are listed under your profile section. Third, upgrade the "Gem Mine" in the Builder's Hall village. Last but not least, you can use redeem codes found inside the "Google Play Rewards" app in available countries.