The Torchwood actor will appear in the HBO fantasy drama's third season, according to WinterIsComing.
Gorman played Owen Harper on Torchwood between 2006 and 2008 and has since appeared in The Dark Knight Rises and BBC drama The Hour.
Submarine actor Noah Taylor is also said to have joined Game of Thrones. The 42-year-old is known for his roles in such films as Lara Croft: Tomb Raider and its sequel, Tim Burton's Charlie and the Chocolate Factory and Almost Famous.
Both actors are believed to be filming in Belfast, alongside Game of Thrones stars John Bradley (Samwell Tarly) and James Cosmo (Jeor Mormont).
Stars confirmed for the next season include Dame Diana Rigg (The Avengers), Mackenzie Crook (The Office), Tara Fitzgerald (Waking the Dead), Tobias Menzies (Casino Royale), Thomas Brodie Sangster (Doctor Who) and Ellie Kendrick (Being Human).
Dominic West (The Wire) recently revealed that he turned down a role on the show, explaining that he was "reluctant to be away from home for a long time".
Game of Thrones airs on HBO in the US and on Sky Atlantic in the UK.
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