keyless entry 99 yukon

Can some one tell me what the gm button is for on the keyless entry for a 99 Yukon???? Is it for a remote starter.or how do you start it with the
keyless entry?What buttons do you push??????It is a 4 button remote
lock unlock horn and a gm button

You have the aftermarket keyless from the dealer installed. The "GM" button is usually for hatch release or remote start if hooked up. You usually have to press it down for 4 seconds for it to work. That keyless is made by DEI www.directed.com