I own Platinum on steam, where it is no longer sold.

Has this cross launcher multiplayer? Does this have anything extra or does Platinum have anything extra?

If I'm correct, the Platinum Edition doesn't include Mysteries of Westgate expansion pack. The version available on GOG does. As for multiplayer, it should work as long as the server doesn't use MoW assets. Keep in mind however that the official multiplayer lobby in NWN2 is long gone. You can access the servers only if you know how to directly connect to them. It's also possible to get the server list working by using Neverwinter Client Extension: https://neverwintervault.org/project/nwn2/other/nwn2-client-extension.