Why does it say to only use that brand coolant? What if I use another? ??

Because GM wants to control every part of your life. No you don't have to use it as if you will read some of the past threads on here a lot of us don't. If you do choose to change to the green stuff and stop using the orange you need to do a complete flush of the system and if you still have that GM rad cap on there throw it away and replace it with a Stant brand cap.

Dex-cool is incombatible with old ethylene-glycol anti-freeze, as the two will gel and clog the cooling system if mixed. I disagree with the stant radiator cap suggestion, stick with OE. Most newer green/yellow antifreeze is now compatible with Dex-Cool, so read the label and you should be fine.

I agree with a different radiator cap other than gm oem, mine broke in to 2 pieces.

I use extended life coolant by Prestone. It's yellow, mixes with dex-cool or standard green. I buy the advance auto parts house brand (same as prestone) Much cheaper than Dex-cool, same protection!