For me, before editing the connection, NetworkManager crashed with a segfault and restarted after attempting to connect with a faulty VPN connection. The same may be what happens to you.
However, I can add a new connection with gnome-control-center or nm-connection-editor (without using any “connection file”; GNOME asks me for the sudo password though) and then I fill in the gateway field. I can then use for example nmtui to activate the connection. I noticed I have openconnect in my config.scm’s system packages, but I believe it is not relevant. I use: (network-manager-service-type config => (network-manager-configuration (inherit config) (dns "dnsmasq") (vpn-plugins (list network-manager-openconnect)))) dnsmasq is probably not required, but I do not know. Regards, Florian