For changing the gateway size, please follow the instruction below:
1. Go to Aviatrix controller -> Gateway -> Select your gateway and click the edit at the top
2. Scroll down a little bit to "Change Gateway Size" section and select the size you want to change in the drop-down menu
3. Click the "CHANGE" button and it will take couple minutes to finish it.
Notice: The gateway resize contain the instance stop and start, which will affect the traffic for couple minutes also.
Refer to: https://docs.aviatrix.com/HowTos/gateway.html#select-gateway-size