E! News said the 31-year-old musician, who is married with a 2-year-old daughter, is the first major rock star to come out as transgender.
Gabel told Rolling Stone he has suffered from gender dysphoria for years and is considering gender-reassignment surgery. He said he will soon begin the transition by taking hormones, undergoing electrolysis and changing his name to Laura Jane Grace.
"For me, the most terrifying thing about this was how she would accept the news," he said of his wife, Heather, to whom he plans to remain married. "But she's been super-amazing and understanding."