Howe didn’t take the striker to Austria for the club’s two-game tour of the country, and the striker, signed from Crystal Palace in 2016, has been training away from the main group.
United’s head coach says he had an “honest” conversation with Gayle – who has two years left on his contract at St James’s Park – at the end of last season.
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“At the end of last season, we had a good conversation, and made him aware of the situation, for his own benefit, so he could take his career into its next phase.
“I was very honest with him. I said from day one, from my perspective, I had a great relationship with him, and I love working with him. He was brilliant for the team last year, and because of all those things, he deserved the utmost respect and honesty from me about the how the future lay for him, and I wanted to give him the best chance to find his next home.
“I really hope he goes on and has a great season next year, and I wish him nothing but success.”
Ciaran Clark and Jeff Hendrick – who were also training separately – have already been loaned to Sheffield United and Reading respectively.
Meanwhile, Newcastle were beaten 1-0 by Mainz 05 at the Kufstein Arena yesterday.
Howe was asked after the game if his pairings offered clues to his likely starting XI for the club’s August 6 Premier League opener against Nottingham Forest.
"No, we've played the same pairings because of the minutes,” said Howe. “So some have played 60 minutes and some 30 (minutes), and then we've just swapped them around.
"The system we're playing, it's always changeable. But that's the system we're playing at this moment and we're willing to change it when we need to depending on the game.
"Don't read too much into team selection at this time."