A:
Long story short... Yes, you would need to buy an Onyx or another SiriusXM radio.
Even though SiriusXM is all one company now, there are three different platforms. The GM satellite radios are XM Radios. There are also Sirius radios and SiriusXM radios. Each of the three platforms have a different channel line up. XM Radios, such as the GM radios, can only receive XM channels. Radios labeled as Sirius radios will only receive Sirius channels. SiriusXM radios will receive SiriusXM channels. Each of the channel lineups have similarities, but also have differences. For example, the Sirius radios have very little play-by-play sports.
The Onyx radio uses the SiriusXM channel lineup, which is the largest lineup of the three.