What is this on my car?

Definitely not factory installed, so who knows.

Looks like some kind of after market switch or monitoring gadget

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It could really be anything. I doubt it's a garage door remote. Who has 4 garage doors? Hardly anyone. It's either some sort of security device or a selector switch for something. Take it out of it's mount and see if there's a sticker on the back of it against the dash where we can't see it and put that info in Google. It should tell you what it might be for. It will tell you what it is and what it can be used for. 

My guess is it might be something like a seat adjustment setting selector. Like when multiple people drive the same vehicle not all of them like the seat adjusted the same so that thing saves their seat adjustment so when they get in all they have to do is press the button their setting is on and the seat goes right there for them instead of having to mess with all the adjustment buttons on an electronic seat. 

Garage Door Opener: Nah. Too many buttons.

Security: Probably not. Doesn't make much sense to have a pin coke thing IN the car. The theif is already in. 

Engine "Tuning" controls: Definitely not. 

It's some kind of selector switch for settings somewhere. What else does a car need different settings for other than the seat?

A garage or a gate remote.

Might be a cheap security device. Enter a number combination to turn it on/off. (with a 15 sec. delay)

Take the screws out (Dismount it) and look for a manufacturer name and model number, then go on line and do some checking.

Since it is not mentioned in the owner's manual, which can be found at https://my.gm.com/content/dam/gmownercenter/gmna/d... it is obviously something that was added.  You can try to track the wiring or ask your mechanic about it.  

Looks like a remote control for a garage door opener. 

Maybe it’s for the engine tune? That’d be my guess.

Clearly a switch. In order to find out what it does you have to trace the wiring to other equipment in the vehicle. It could be settings for ride height, for example.