To get your default gateway IP, show your routes with the
ip route list command.
Edit your main internet connection to remove the default route: in
nm-connection-editor, edit your connection > IPv4 tab > Routes > check Use this connection only for resources on its network.
Then add a specific route to reach your VPN server via your gateway. This will allow your computer to reach only one IP on the internet: your VPN server, so that your VPN connection will still work. (in the Routes window, add a route like:
Adress: VPN_SERVER_IP Netmask: 255.255.255.255 Gateway:GATEWAY_IP).
To allow internet access through your VPN, you may also need to edit the Routes of your VPN connection to uncheck "Use this connection only for resources on its network", or add a specific route for a custom gateway of your VPN.