I'm afraid not. The online mode was developed with help from Valve or Steam developers and they apperently couldn't be asked to put in the extra effort it would take to do it on GOG Galaxy. As a sorry excuse, they upped the price of the dlc from the regular 4,99 to 5,69 euros on Steam and pretended GOG got a discount. I love the game, but this was quite scummy.
GC13: So what you're saying is that the Steam version has something extra and costs extra money as well? Pardon me but that doesn't sound scummy at all.
I think what he meant is the following (Current listing):
GoG (CAD)
Base game + Nyakuza Metro: 19.99 + 5.99 = 25.98
Steam (CAD)
Base game + Nyakuza Metro + Online: 16.49 + 7.99 = 24.48
I don't see a discount on GoG's side but, I'm using CAD and as they mentioned in their blog they got help from Valve for stream MP integration. If anything, it's sketchy the other way around, mind you, I bought this on GoG two years ago because of a sale. I didn't regret it in then and I definitely didn't think there would be any disparity between PC copies unlike Monster Hunter World. (PC port released later)