mqstout: Grim Dawn does not require Galaxy to play SP, including for LAN-based multiplayer. It does accept Galaxy for Internet multiplayer (if you don't want to use a VPN connection or such). Grim Dawn's Internet-based multiplayer always required middleware. Fortunately, the awesome devs at Crate made sure it still had direct connect (LAN). [I was a crowdfunding backer, followed development, played immediately on release, etc.]
Syphon72: Don't accept "most games do" as an excuse. Even if it's true, it should still be disableable and be opt-in. It's been ages since I've re-installed a fresh GD copy (I've been steady with keeping it), so I don't remember if it was opt-in, but it sure is easily toggled. But, as was said, there's a difference between a specific game doing things internally, and external software doing things.
OP, there's a major difference between Steam and Galaxy for multiplayer. There's even arguable reasons that Galaxy-multiplayer is better than "each title does their own Internet multiplayer" (for future deprecation and development of drop-in replacements). Fortunately in Grim Dawn's case -- which I wish were universal -- there's LAN play also included.
More games need to support LAN or re-work the game once its old & not many are playing to support it.
Especially in an era when MP can die out pretty fast. Feel bad for all the dev's putting their blood, sweat, tears, and pixels into all of this, making these awesome competitive COD-style MP-portions...only for servers to get pulled later b/c nobody's playing anymore. LAN/TCP-IP support solves that.