My iPhone is hooked up to my home wifi. Am I also using my GBs when I use my home wifi for Internet?

No.

Of course not unless you enable Gbs either you are connected to internet over wifi or 3g , for accurate directions for how to do this go to user manual of your iphone model then go to wifi and Gbs section, i hobe that will help you to know more .

The phone defaults to Wi-Fi for data transfer when Wi-Fi is available. The cellular connection would be used for SMS, MMS text messages and phone calls.