Did an inspection in a house w/older gas furnace; when i got to the thermostat it listed as “for electric furnace only”. Why gas furnace with for electric furnace only or i am making something out of nothing? Appreciate any help.

Probably has to do with the heat anticipator the amp draw of the gas valve should match the thermostat anticipator if it has one. That old gas furnace in your pic would have a 24 volt gas valve, no milivolts they were used on floor furnace gas valves. If the furnace cycled on and off with the thermostat I would not be concerned.

Electric furnace thermostats are set up to energize the fan relay on a call for heat as well as call for cooling. If there is no air conditioning then the fan circuit would not be wired in.

I am more concerned with the staining around the burner cover indicating that there is a possible venting problem or a possible leak in the heat exchanger causing combustion gases to spill slightly when blower comes on.

So noted. Scorning was noted; aswell as corrosion on the vent pipe; serious issues were brought to the attention of the client. Thanks!