This kind of cross-pollination happens sometime in fandoms with popular, well-done stuff. Build your empire, research technology, command mighty warships, engage in diplomacy, make allies and trade. Choose your race as the Tau'ri, Asurans, Goa'uld, Wraith, or Asgard. A couple of their designs also seem to take cues from his work (compare the rear/sides of his Ares and their Claudius). This Sins of a Solar Empire modification brings the various Stargate franchises into the fold. The design is also on the cards to be used by the makers of a mod called Stargate Invasion for Sins of a Solar Empire. He's got a whole load of work on there and was active on a range of forums and showed his workings out as he'd create stuff, much of which is also posted on there. Mallacore/RJB's deviantArt page can be found here. The BB-307 page has a note at the bottom that the art is by 'Mallacore.' The links above are for the individual ship, not the class it belongs to - the difference between the Prometheus and the 303. That isn't an artist's mockup of a ship from that forum game, the forum game took an artist's work and incorporated it into their thing.

The other answers are right in that it's fan-made and used in the game Stargate: The Next Generation but that's not where it comes from.

Sins of a solar empire stargate