Not knowing GMC trucks, I assume when you say electrical temp sender it is not a bulb type setup.
To address the sender first: To start with, a temp sender does not care about the battery voltage, so 6 or 12 doesn't matter. Your problem is what they call the temp/resistance response curve. The sender changes resistance as its temp changes. You will need this curve in order to buy the correct sender unless use your old one. You can derive this curve with either of two simple tests if necessary.
The gauge is a different matter. I believe it is polarity sensitive but may be wrong. If it has three electrical lugs on the hookup side it should be no problem. I am again assuming that your will be using a 12 volt negative ground setup.
