My Apologies, I assume that all classics would be the same, but not quite sure what fuel Im running would have to do with my temp gauge not working?That would depend on year and model. As we have no clue what continent you are on we can't even narrow it down to what fuel you could be running.
Test gauge as migget says and make sure connection is tight & clean. Then do yourself a favor and install a real temp gauge like a VDO.
Here's the test sheet for temp gauge but I think it is for the sender to the ECU not the gauge.... might be similar though.