The 24-year old striker is a target for Norwich City this winter but McGarvey hopes he will stay at Parkhead and help the Bhoys in their Champions League Round of 16 contest with Italian giants Juventus which kicks off on February 12.
"Gary is only 24, he is scoring goals in the Champions League and the Scottish Premier League. He can do no more than he is doing," McGarvey told Record Sport.
"He could be getting a call-up into the England squad soon because he is scoring goals in the Champions League, which is the top league in world football. For English clubs to offer £5m is an absolute joke."
The former St Mirren and Liverpool man added: "I wouldn't go to Norwich. But if you are doubling your wages it would be very difficult to say no.
"Who in the world would say no to doubling their wages?
"They have held on to their players well in the last couple of years and have made more than £20m from their involvement in Europe because of that so I think they will try to hold on to Gary.
"If he scores against Juventus and helps Celtic into the last eight, that would make them another £5-6m. You have to look at his agent, he will be egging him on to go because that makes him money. He will be getting pressured from both sides.
"If Manchester United came in and offered £15m then I would think about that but if he stays at Celtic he will be in the Champions League every year and pit his wits against the best players in the world."