[2021] Gems

We assume by gems you mean Diamonds?
The best source for free Diamonds is the weekly Spire of Eternity, which has up to 500 possible each week. This figure is made up of individual endeavour and Fellowship participation in the Multiplayer aspect.  

@Hevda there are sometimes Diamonds in the chest in the Magic Academy and in the chests above the Advanced Scouts research in your Research.