Is the co-op in this similar to how Gauntlet was?

Do you mean couch-co-op? 2 or more guys at the same machine? No....everybody need his own PC for Grim Dawn.
Co-op is means that you can play together in a local area network (LAN) without an online-connection.
If you want to play couch-co-op you can play Vctor Vran or Vikings-Wolves of Midgard for example.

I have 2 machines set up for when friends come over. It's sad that there is no local co-op as thats the most effiecient when playing with friends.
I can play it online with other computer in same house. Was just wondering how the gameplay style for co-op is like? Wondering if its like 1980's Gauntlet game or has a different play style?

It is like a normal hack'n'slay game. You run together through an open world and kill monsters and get loot and xp. You can also play the main story together. In my opinion it is the best hack'n'slay game if you want to play with a gamepad. But you can switch to mouse and keyboard everytime.
In my remember gauntlet has a level system and no open world....that is the diffrence. Buy it and you will love it :D

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