Q. Can I use gateway transit for a virtual network that has its own gateway?
A. No. Gateway transit enables a virtual network (e.g. vNet A) that is peered with another virtual network (e.g. vNet B) that has a gateway to an on-premises network to be accessible via the on-premises connection, e.g. on-premises can communicate to vNet A via its connection to vNet B. Note this is only possible if vNet A does NOT have its own gateway. With VNet peering you can only have a single gateway that can be traversed.