There are some differences. Invasion is about survival, persiod.
Gauntlet had the portals that you had to destroy. You guys also had custom monsters too. That makes it unique. There aren't portals in invasion.
The fact that they did this should make Gauntlet a possibility for you guys since you don't have to figure out how to get the monsters into the game in the first place.
Also, Gauntlet was unstable as all hell on my system. Invasion isn't.