Hellgate: London combines the depth of role-playing games and action of first-person titles, while offering infinite playability with randomly created levels, items and events. The player creates a heroic character, completes quests and battles through innumerable hordes of demons to advance through experience levels and branching skill paths. A robust, flexible skill and spell system, highly customizable appearances and a massive variety of randomly generated equipment allow players to create their own unique hero.

    Hellgate: London was designed to be primarily focused on solo and cooperative PvE combat, but players can duel and there is a free-for-all PvP Mode for subscribers. Dueling can only take place outside of Underground hubs. Players can also choose to enter into PvP mode which means they can be attacked and harmed outside of Underground hubs by anyone else that has chosen to enter PvP mode. That is, those in PvP mode, must always be ready for PvP. This is a way to have wide-ranging free-for-alls, or create a “friendly-fire” way of playing the game. In multiplayer mode, players can meet and organize for team play and quests in safe zones - the old Underground stations, protected by the Freemasons' wards. The world will not be split in "shards" or servers, but rather play like a massively multiplayer online game with heavy instancing, such as Guild Wars. Every character is capable of soloing the entire game. Grouping with other players is optional, though grouping will bring benefits in terms of experience gain and items. Difficulty will be balanced where the opposition grows stronger as more players play in the instanced areas.

The game includes three different quest types:

• Storyline - Special "scripted" quests following the storyline tightly and moving it forward. Key quests are shared here, Storyline quests will be highlighted in orange color in the quest window for clear identification. Storyline quest givers also have a blue marker over their heads.
• Template - Straightforward, built off eight basic formulas: collect, explore, escort, infestation, hunt, travel/talk-and-do, use item, and operate object. Their primary function is to give players something to do while they undertake story quests.
• Task - Generic quests believed to have more random objectives, given by NPCs out in the game world. Completing these quests increases one's reputation to unlock class-specific quests, as well as revealing new items and giving experience for new levels. Many tasks are in fact randomized, and the NPC may or may not appear in a given game. Some tasks are harder to find than others.

    The weapons base their power on character stats more than a player's aiming skills, being more of an RPG than an FPS in this regard. For example, a player can place points in an Accuracy statistic that determines the randomness of the rounds they fire.

    Each weapon type does have a very different effect. For example, the Grizzly Rifle fires a stream of energy that causes splash damage on impact; the Rebounder fires elastic energy balls that ricochet off walls; and the Cabalist-only Locust Hive is a short ranged rifle that causes a swarm of insects to sting and damage the target over time. Melee weapons are less varied, but there are swords that can inflict toxic poison or explode with splash damage on striking.

    Besides the behavior, the weapons, close and ranged combat alike, can do several different kinds of damage. Each damage type also imparts a special negative effect on a critical hit:

• Physical - Can cause enemies to get stunned.
• Ignite - Can set targets on fire, draining 5% of their total life per second.
• Spectral - Phase shifts targets, causing them to deal 50% less damage and take 50% more damage.
• Toxic - Poisons the targets, making them suffer damage over time and unable to be healed during the poison's duration.
• Electrical - Can make a target unable to use skills.