A copy of Garruk Relentless cannot transform. Even if you copy it with a transforming DFC, the copy effect will still apply and overwrite the other side's characteristics, so will stay the same if transformed regardless. If you have such a thing and damage that doesn't kill it is done to it, the state trigger to transform will go on the stack, and when it resolves and fails to transform the permanent, it will go on the stack again, ad infinitum. This is a mandatory loop and thus a draw if no player can or wants to stop it. So the game ending in a draw is correct.

Note:
A token copy of Garruk Relentless will be a transforming double-faced permanent and can tranaform. But a token that becomes a copy after it entered the battlefield will have the same issue as any other clone. Because a token created as Garruk has Garruk's base characteristicts, while a token created as something else and then turned into a Garruk will have a copy effect applied to it overwriting the base characteristics.