The store page doesn't mention it, so I'm guessing it's not included in this release, but just to confirm does the GOG release support the co-op/1 vs 1 multiplayer? It was arguably my favorite part of the game, I sunk dozens of hours into it on the Steam version.

It's not included. There's some fine print below the system requirements: "NOTE: This version of Blades of Time does not support online features"

Thank you, I didn't catch that.