The 26-year-old has only been playing the word game for seven years, but took hom the £2500 prize after beating Wale Fashina in the final at Cavendish Conference Centre in West London.
“It feels surreal,” he told the Daily Mail. “I am delighted. It is something that I have always wanted to win ever since I started paying Scrabble.’
Gallen is the first winner from Northern Ireland in the competitions 41 years.
He said he memorises unusual words but doesn’t take much interest in their meaning.
“It is really a game of maths – you are just taking on extra work by trying to learn all the definitions,” he said.
He won Countdown’s Champion of Champions in 2006
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