Hey Mate,
i did the same. Told a Friend to buy Grim Dawn finally because i saw the GoG Offer.
So at first we had the same Problem with Steam/GoG Multiplayer. But it works definitely with Hamachi.
Here what helped us:
[ul]
[li]right click on Hamachi in the Network Connections window, and click Properties in the drop down.[/li]
[li]On the first window that pops up, click Internet Protocol Version 4 (TCP/IPv4) in the box in the middle of the window, and then click Properties to the bottom right of that.[/li]
[li]On the next window that pops up, just click Advanced in the bottom right.[/li]
[li]Uncheck “Automatic metric” and enter in the field below “10”[/li][*]choose the same window from the default gateway entry (usually “5.0.0.1”). Remove turn uncheck “Automatic metric” and enter this time the value “1500” in the field below (2). Click “Add” and confirm all changes with “OK”.[/ul]