Assuming your refrigerator is an auto-switching model, it's detecting the availability of 120v power, switching off the gas and failing to cool on 120v.
I'd say that the most likely problem is that the 120v heating element has failed, since the 120v input needs to be present to make it try to switch away from gas. In the short term, you should be able to set the refrigerator to gas-only operation. The electric heating element can be replaced, if tests find that the element itself is the problem. Hopefully someone with more detailed knowledge of auto-switching fridges will reply, but there are several useful threads on here about the electric heating element.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill