Players will take on the role of a wandering mercenary with their own special set of skills and their own reasons for traveling to this remote corner of the world. Players must work together out of necessity to clear out menacing dungeons and forgotten ruins. In the process they will enhance their abilities with experience and loot, discover new locations to explore and plunder, and expand an ever-branching story fueled by the decisions they make. This is a persistent game that is intended to be played over many game sessions. After a scenario, players will make decisions on what to do, which will determine how the story continues, kind of like a "Choose Your Own Adventure" book.
Playing through a scenario is a cooperative affair where players will fight against automated monsters using an innovative card system to determine the order of play and what a player does on their turn.
Essentially, every turn a player will play two cards out of their hand. Each card has a number in the center, and the number on the first card played will determine their initiative order. Each card also has a top and bottom power, and when it is a player's turn in the initiative order, they determine whether to use the top power of one card and the bottom power of the other, or vice-versa. Players must be careful, though, because over time they will permanently lose cards from their hands. If they take too long to clear a dungeon, they may end up exhausted and be forced to retreat. I've written a comprehensive series of blog posts about the inception and development of Gloomhaven. If you're interested in knowing more about the game, please read on!
You can find a lot of resources for the game on the Board Game Geek page, including glowing reviews and extra fan-made content.
You can also play the game on Tabletop Simulator.
I also wrote a lot of other contemplative, pertinent posts about the development of Gloomhaven, so check those out as well.
Phew! I think that's just about everything!