There are different laws at play here.
In thinking about the CPL laws, GLC is not a no concealed carry zone (unless there is a "bar" or other restricted area within, then that specific business would be off limits for concealed carry).
However, trespass laws also apply. If a private property owner/manager puts you "on notice" that weapons are not allowed (either by a posted sign or by verbal information), you will be in violation of trespass laws if you do not leave the property. Technically, if the private property owner/manager push the issue, they could claim that you were already "on notice" via the posted signs/rules, but in common practice, you are usually verbally put "on notice" and asked to leave. Just remember, "usually" does not mean "always".
Also, if you are faced with using your firearm in otherwise legal defense, the fact that you are someplace that you "shouldn't" have been with a firearm will surely be used against you in court.