I didn't quite get your specific doubt, but lets say you have an overlay segment on network 192.168.1.0/24 connected to T1-A with IP 192.168.1.1 as default gateway.
Now you have a physical server on VLAN 10 on subnet 192.168.10.0/24 whose default gateway is 192.168.10.1. You can configure T1-A to be the default gateway of VLAN 10 with the following steps:
1) create a new segment on a VLAN transport zone that is available on the edge nodes and configure it with VLAN 10;
2) edit your T1-A gateway and add a Service Interface;
3) configure this interface with 192.168.10.1/24 IP address and connect it to the segment created on step 1.
With this you have a T1 that on one interface is default gateway of the overlay segment and on the other is the default gateway of VLAN 10.
Now all you have to do is configure gateway firewall rules so that VLAN 10 can only access what you want.