I am not familiar with Garellis but I will jump in here. I am sure there will be a better reply to follow....
Most mopeds have some kind of arrangement to disconnect the engine from the drive chain.
On some there is a button on the rear hub that you can push to disconnect things.
On others there is a lever on the rear hub or at the gearbox that you can operate to disconnect.
Do you not have the owners' manual? It should be stated in there about how to free the engine from the drive chain. If you do not have the manual you can probably download one from the 'net.
Perhaps there is a lever on the handelbars that you can pull to disconnect the drive.