Gloomhaven is a series of tactical combat scenarios that are strung together in a persistent, thematic campaign.
Each scenario is played out through rounds of combat against fully automated enemies. In each round, players will play 2 cards from their hand of ability cards which determine the order in which they act and what abilities they perform. Each card has 2 special actions, and on their turn, players can play the top action of one card and the bottom action of the other.
Card actions are varied and powerful, cutting through the monotony of a basic "move and attack" system. Default actions are available, though, if a player's plans get ruined by an enemy's ability card or even by the actions of their own teammates. Gloomhaven eliminates the alpha player through simultaneous hidden action selection to illustrate the chaos of battle.
There are also no dice in Gloomhaven. Instead, attacks are modified by a player's attack deck. Over time this attack deck will change as a player begins to add and remove cards by completing battle goals and leveling up.
Players gain the experience needed to level up by performing certain actions in their ability card deck. Through leveling up, players will also get access to more powerful, higher level ability cards which they can use to customize their deck.
In midst of battle, players will also need to focus on looting money tokens. These can be used to purchase powerful items in town, where new stock is always being unlocked by player actions.
One way to unlock items is to increase the prosperity of the town, which players can do by completing scenarios and making decisions during events between scenarios. Players will also have to manage their reputation during these events, as reputation will influence the outcome of some events and also affect shop prices.
As players interact with the town and move along the branching story paths of the world, they will always be working towards completing career goals. By meeting certain objectives, a player's character will retire, unlocking a new character class to play with, new events for parties to encounter and additional perks for the achieving player. After retirement, a player will create a new character, possibly from the class that was just unlocked, and continue the adventure!
If you'd like more information about any aspect of the game, you can find a wealth of information on the Gloomhaven website and also the BoardGameGeek page which features a number of illustrated lessons on how to play the game.
In addition, here is a PDF of a preliminary draft of the rules.
If you learn through action or just want to get a better feel for the game, there are a couple easy ways to sit down and play the game on your computer.
The introductory scenario of Gloomhaven is available through Tabletop Zen (free) and Tabletop Simulator (this requires the purchase of software).
You can get the Tabletop Simulator Steam Workshop mod here. From there you can play around with it on your own, or sign up to be taught the game through this Google Doc. When you become familiar with the game, you can also sign up for pick-up games, or even be a teacher yourself.
Tabletop Zen now has an open lobby where you can go play around with the game on your own, or you can sign up for pick-up games or teaching games through the Google Doc. All that's required is a valid email address to sign up.
Keep in mind that the game is not automated. Players will have to move the pieces around according to the rules, and communication between players is highly advised. Tabletop Simulator has accompanying voice software, but Google Hangouts is needed to play on Tabletop Zen.
The scenario available online is just the beginning, though. A scratch on the surface of the world.
Gloomhaven is deep and expansive. It is a game of exploration and discovery; of branching story paths and decisions with real consequences. You always have a choice of which scenario to undertake next and which plot thread to unravel.
More than that, though, there is plenty of content to support multiple game groups. If you want to play the game with someone new, you shouldn't reset the game, but instead just pursue a different story line. Each party of adventurers will develop their own campaign, but everyone will interact with and expand upon the same world.
Gloomhaven provides a persistent world of permanent choices without the restriction of only being able to play with the same people time and again. It does this by focusing on the stories of individual characters instead of an over-arching linear plot.