The GOG version only supports Direct Connection and LAN for multiplayer, not the online lobby.
If you had a working virtual LAN using Hamachi then the game should have shown up if the host had their 'LAN Connections' visibility in the Game section of the options enabled, and the client clicked on Join from the main menu and (if required) then the LAN button at the top of the window.
A Direct Connection can also be made there, if the client has the host's IP address or Server ID (which the host can get after starting/loading a game in the Connectivity Menu, available by hitting Esc to bring up a menu or clicking on the diamond icon at the top of the mini-map).
