Hello @Joman Sierra ,
That is the issue.
You can configure the gateway in the peered virtual network as a transit point to an on-premises network. In this case, the virtual network that is using a remote gateway can't have its own gateway. A virtual network has only one gateway. The gateway is either a local or remote gateway in the peered virtual network.
If both the Vnets have their own VPN gateways, you cannot enable "gateway transit" option.
The only workaround in this case would be to delete the P2S VPN gateway from "Resource Group A" and then configure P2S VPN on the already existing S2S VPN gateway of "Resource Group B". Then peer both the Vnets with the option "Use the remote virtual network's gateway" (on the peering of Resource Group B vNet) and Use this virtual network's gateway (on the peering of Resource Group A vNet). After this, you can access the resources from both Resource Group A and B from the P2S client connected to Resource Group A.
Regards,
Gita