The only way to add a VLAN in Gnome now is to create it through nm-connection-editor
A few years back Gnome removed VLAN config from the Gnome settings section, now the only way to create new VLAN config is via nm-connection-editor, which has to be run from the terminal.
I would appreciate it if you could allow us to add VLANs via the Settings > Network page, it's a useful feature if you have multiple VLANs that you need to access via tags
I am using: GNOME Shell 3.28.1