Gartside told The Mail on Sunday: "I've already decided that if Freddy Shepherd wanted to talk to Sam about the Newcastle job he would be denied permission to do so.
"I've explained my thinking to Sam and I've already told him that he wouldn't be allowed to speak to Newcastle at the moment under any circumstances.
"He accepted my decision and he is fully committed to Bolton Wanderers.
"The only way I would be prepared to consider allowing Sam to leave the club - and he has three years left on his contract - is if the England job came up."