The 22-year-old has signed a five-year contract with the Magpies.
Newcastle manager Alan Pardew told the club’s official website: “Gabriel is a player I’ve watched for the last couple of seasons since he arrived at Manchester United.
“He has great pace and excellent technique and he has improved his power in the last 12 months and is ready now to make a real impact in the Premier League.
“I contacted Sir Alex about him early at the start of the summer to see if there was an opportunity to bring him to Newcastle, but it wasn’t until Wayne Routledge joined Swansea that we were able to bring him in.
“His best years are ahead of him which is exactly what we want at this club, and with the backing of our magnificent fans I’m sure he will prove an excellent signing for us.”