The "C" connection is the common or ground on the 24 volts side of the transformer. It should have zero volts on it. It should not have 24 volts or 120 volts.

I found a link which I believe is the wiring diagram. Page 27-28 show a C connection with blue wiring (it us usually black). See if you can find this connection and verify it has zero volts.

Here is a link that might be useful: GPG13 wiring

Although this post is almost two years old, I hoping you'd be will to share how you solved your problem. I have this same exact setup, a goodman gpg13 and thinking like you I figured the extra blue wire in the block was my holy grail of "c" power, unfortunately my meter tester at the thermostat is telling me different. Thanks for your help.