According to the information I have, the radio in your particular car can either be satellite ready or not satellite ready.
What is the model of your Delco radio? Do you have a photo?
Well, that's what we're trying to ascertain.
Can you pull out your owners manual and give me the model number of the Delco radio in your Buick.
According to the installation instructions, the radio has to be sat-ready.
What the GM1 does is replace the factory XM receiver unit that GM puts in the car so that people can get Sirius. If you didn't have XM, your head unit may not be sat-ready. I don't think yours is.
You could try getting a standalone Sirius unit but of course you wouldn't be able to control it from your radio.
Shadetree Mechanic
Chevy nut and Suburban owner