Michigan doesn't have provisions for "no CCW" signs in the law. The actual wording of the Michigan CPL law with regard to pistol free zones is: "An entertainment facility that the individual knows or should know has a seating capacity of 2,500 or more."
I guarantee you that the GTC has a licensed seating capacity in excess of 2500 people based on it's square footage. The GTC has hosted other events in the past, including boxing and wrestling, so I think that most people would/should reasonably know that the facility will hold at least 2500 people in its 50,000 square feet.
All that I'm saying is that, personally, I wouldn't risk my CPL by carrying in the GTC. Their request at the very least makes it trespassing if they catch you resulting in removal from the facility, and at the worst, you get busted by the police for violating the CPL law.
I know that at the Grand Rapids gun show, they actually stat in the "no concealed pistols allowed" signs that the facility has a seating capacity in excess of 2500 people. Does the GTC signage say the same thing?