Gloomhaven is, at its heart, a puzzly dungeon crawler. The board game that this is based on relies on several subsystems all working in tandem to subvert mechanical tropes, enhance player choices, and force the player to make impactful decisions. The Early Access to the electronic version of the game is heavily built on promises; the road ahead as laid out by Asmodee will have the video game slowly align with its board game inspiration. We're not there yet, though.
Gloomhaven the video game, right now, is a one-player dungeon crawl where a party of two out of four possible characters trudge through what is the most nakedly bare-bones gameplay loop you're likely to see for a game with this kind of pedigree. It currently offers only a roguelike mode, exclusive to the video game version, which has you choosing one of three directions to travel, with a location to reach. Each of the three paths has a different difficulty level and a number of unavoidable dungeons to raid, the more difficult of which you're unlikely to clear until you've undergone a sufficient amount of grinding (though even the easy routes will get tougher the more you level your characters).
Adventures earn you gold coins and equipable items, if you've picked them up before the last monster is killed (at which point the level concludes at the end of your turn). Items have durability ratings that decrease when you reach a location, with zero durability causing an item to disappear even if you've never equipped it. Gold is spent on items should your destination have a shop. Apart from events that may happen in transit to a dungeon, there is little to no flavour to the game beyond the varying abilities of enemies and characters. Levelling means picking a new ability card (more on these later) which can be switched out for an existing card, and getting an extra potion slot every few levels and a few more hit points. A tough pre-set boss scenario needs to be beaten in order to open a character slot which allows a party of three, then four.
Gloomhaven the board game has a campaign mode, allowing several players to work together. It has hidden achievements and goals for each character, and finishing dungeons unlocks permanent changes, culminating in characters retiring, making their mark on the world and unlocking new characters to play. There is a starting roster of six possible characters, with even more unlocked over time. Levelling includes permanently enhancing your ability cards with extra effects and decreasing detrimental effects, or increasing special effects in a deck of damage modifiers unique to each character. Campaign quests allow players to choose what they will do next, and at least some flavour text contextualises one's murder-hobo adventures. Though it's arguably unfair to compare the unfinished video game to the completed board game, it's something the creators readily do, as the board game is basically their end-goal.
Everything is so finely tuned in the board game version that you are made to feel much more invested, both in your character and the world. Asmodee has promised an approach to the electronic version that will eventually try to duplicate the board game, but even then there are things a board game permits that fly in the face of traditional computer game design. Partial taking back of a turn because you didn't understand a rule or modifier is impossible, so if you step on a trap that was obscured by an urn? Tough shit, even though you would have known it was there if you'd placed it in the map on a physical board. The wealth of different, interweaving factors in a given turn may leave a player overwhelmed but the game relentlessly pushes things forward; rather than players misinterpreting the rules but still having a fun time, the computer knows the rules and applies them with merciless efficiency, and you can easily waste turns on misclicks or hitting "skip" when you meant to follow through. By divesting the player from the mechanisms, the experience is necessarily different, even if mechanically it's identical. The discovery mechanisms in the board game, too, are unlikely to feel quite the same, especially since players are often expected to know more of what they're getting into in a video game, with a character's secrets often laid bare to start with. These sorts of things are largely unavoidable with an automated board game, but they're still worth mentioning given how complicated the game can be.