It's a Braha'tok-class gunship.
They've been used by the Rebellion since the start of the war, and were seen in Star Wars Rebels (Season 3 finale), as well as Episode VI - Return of the Jedi, just before the Battle of Endor.
The Braha'tok-class gunship, also known as the Dornean gunship, was an anti-starfighter gunship constructed by the Dornean Braha'ket Fleetworks Conglomerate for use by the independent Dornean Navy. Designed purely for military use, the ninety-meter-long craft bristled with weaponry, including turbolasers and concussion missile launchers. At first, the gunships were used to defend Dornea from the Imperial Navy, but in 4 ABY, two of them were sent to Sullust to join Admiral Gial Ackbar's Rebel Alliance Fleet, which was poised to strike at the Galactic Empire's second Death Star over the Forest Moon of Endor.