I used to do this back in the day when I used my AR750 Slate as a client to connect to my NetGear Router VPN server. I added the static IP address and gateway to the client config file before importing the file into the AR750 router’s client config.
route-gateway 192.168.0.1 ifconfig 192.168.0.254 255.255.255.0
So for GL.iNet as the OpenVPN server, this might work.
route-gateway 10.8.0.1 ifconfig 10.8.0.254 255.255.255.0