To get Hogwarts Mystery Free Gems for Hogwarts Mystery, you must find a place called Monster Hunter in the game. This place is quite difficult to find. I myself stumbled upon a Monster Hunter by accident - click on the picture my Harry Potter fell into the picture. You can visit this place only once a day. Free Gems I could get only after I defeated the monster (each time the monsters are different). For the victory you get from 100 to 1000 free gems. Winning a battle with a monster is very difficult, but I did it the first time.
Look for the Monster Hunter oddly enough on the top floor. My picture looked like a ghost with a pumpkin head. I think this place will soon disappear as it happened on Halloween night. So if you want to get up to 1000 free gems, you need to hurry to find the place of Monster Hunter.
If you failed to find Monster Hunter, then you can use another guide that automatically generates you much more free gems.