It's tough to make a single suggestion because Gloomhaven does a lot of different things and puts them all into a single package. I don't feel like there is any one other game that does everything Gloomhaven does, but there are plenty of good games out there that replicate some aspects of it but not others. So it kind of depends on what aspect of Gloomhaven most appeals to you.
If you're looking for an RPG-in-a-box then Folklore: The Afflicition is good. It was also just reprinted from a second Kickstarter so copies are relatively easy to find and inexpensive. It's not quite Gloomhaven though because the RPG elements are stronger, while having less emergent gameplay and more randomness through the use of dice.
If you're looking for card-based gameplay that can be played solo, co-op or competitively there is Mage Knight. Or if you prefer a sci-fi theme there is a re-skinned Mage Knight named Star Trek Frontiers that I prefer. There's no campaign mode though as each game is an isolated scenario. Another great card-based co-op game is Spirit Island, but it again doesn't have a campaign mode and is only similar in terms of the gameplay elements.
If you're looking for a good story Mansions of Madness is a solid choice. The mechanics are nowhere near as tight as Gloomhaven though and there also isn't any campaign or character progression.
If you're looking for a thematic dungeon crawler then there are tons of choices. Descent and Imperial Assault both have apps that make them coop now and they're both solid choices as they have campaign modes like Gloomhaven has. That's where the similarities end though because they rely much more on dice chucking and are far more Ameritrashy than Gloomhaven.
"For a long time now I have tried simply to write the best I can. Sometimes I have good luck and write better than I can." - Ernest Hemingway
Well, that's an excellent answer and I myself took to a couple of hours research to find out, only for the wife to come home and mention that she has, through her network of social contacts, managed to snag me a remaining copy of the game from a darkened sorcerous shop somewhere.