I have not found any other brand's wheels that fits directly. Making other wheels fit your bike is often a quite work-consuming task. I have done it (not to my Gasgas), but you often need a lathe to turn some new parts. Often you will have to move either the disk or the sprocket (or both) sideways. Disk sizes can also be an issue.
On the other hand, Gasgas wheels of almost any year will fit without other modifications that just a swap of the bushings/distances. I have a 06 and a 15 model, and use those wheels on the 15 bike without modifications. I know that when the 2018 bikes arrived, people used their older wheels on them, only adding a thin spacer on one side of the rear wheel. I don't remember on which side, though. But my point is that you should be able to use older GG wheels without much trouble.