Warhammer 40K: Chaos Gate - Daemonhunters is the latest game set in the Warhammer 40K universe and set to be canonical within the world as one of the many campaigns against chaos. The canonicity of games within the Warhammer 40K universe is often ambiguous on purpose. With the exception of a few set events, Warhammer is set up so that any battle could be a possibility within the lore, and Daemonhunters is no exception. However, it is possible to speculate where this game fits into the canon, based on the few details that have been given about the setting.
Warhammer 40K: Chaos Gate - Daemonhunters follows the story of a regiment of Grey Knights as they are brought in, fresh off a campaign against Khorne, to fight against the plague hosts of the pestilent Chaos god Nurgle. Warhammer 40K: Daemonhunters contains the turn-based tactical RPG combat reminiscent of modern XCOM games. Lore-wise, the Grey Knights are a chapter of Space Marines, or Astartes, that are created from the gene-seed of the Emperor Himself. All Grey Knights have potent psychic abilities and are immune to Chaos corruption entirely, a rare trait in the Warhammer 40K universe. For this reason, they have become the militant branch of the Ordo Malleus, an inquisitorial sect that is charged with defending the Empire of Man from Daemons and Chaos. In this capacity, they are the eponymous Daemonhunters, who charge themselves with fighting any and all daemonic incursions.
At this point is where the plot of the game comes in. The Grey Knights that the player controls have been drafted into another daemonic campaign by the Ordo Malleus and the Grey Knights are duty-bound to answer. While the game is ambiguous on when it is taking place, there are some details provided when the Warhammer 40K: Chaos Gate - Daemonhunters release date was announced. These details can be used to speculate on where the game takes place in relation to the established canon of the Warhammer 40K universe.
The Grey Knights Were Hidden for Most of Warhammer 40K History
The Grey Knights are a relatively new chapter of Space Marines in the lore of Warhammer 40K. Even still, the Grey Knights have been around for thousands of years within the timeline. Originally, the Emperor of Mankind created the Grey Knights directly. Shortly before the Emperor's death during the Horus Heresy, Malcador succeeded and gathered those he had chosen on Saturn's moon Titan. The founding , alongside thousands of recruits, were then psychically sealed away within the Warp, leaving their existence known only to Malcador and the Emperor.
After their emergence from the warp, the Grey Knights were unleashed to perform the task for which they were originally created, protecting the Empire of Man from the daemons of Chaos. With their unique Warhammer 40k Space Marine training and powers, they are uniquely suited to fighting the Ruinous Powers. Within the lore, the Grey Knights are kept secret, often treated as myths and legends to conceal their presence within the Astartes legions. After their emergence, it was decided that the Grey Knights would become of the militant arm or "Chamber Militant" of the Ordo Malleus, itself equally secretive in its existence. Thus, the Daemonhunters of the Empire were formed.
As this is where the name of the game comes from, it stands to reason that the game would have to take place well after the events of the Horus Heresy. The emergence of Titan from the Warp occurred sometime around the time of the Second Founding, around 021.M31 in the Warhammer 40K calendar. Therefore Warhammer 40K: Chaos Gate - Daemonhunters would have to take place after that.
Vardan Kai's Presence Suggests a Recent Setting in Warhammer 40K
Warhammer 40K: Chaos Gate - Daemonhunters has a host of characters to interact with. One such figure that can be latched onto for reference is Vardan Kai, the "current" Grand Master of the 1st Brotherhood of the Grey Knights and the Steward of the Armory. Vardan Kai, voiced by Andy Serkis, is present in Warhammer 40K: Daemonhunters where he acts as an adversarial force that the player must engage with to procure new gear and equipment for their Space Marines.
Vardan Kai, as mentioned, is among the most current known Grand Masters of the Grey Knights, placing the setting to be sometime within the most current lore. Trends among games and books produced by Games Workshop for the Warhammer 40K universe tend to move in a progressive manner, with each looking to update the lore to the most current timeline. While the exact dates are near impossible to figure out, these facts suggest that the newest game is likely the newest entry into the lore as well, making Warhammer 40K: Chaos Gate - Daemonhunters the most current canonical entry.