Where can I find resources on Gas Furnace manufacturers
Are you looking for reviews or just general information about the furnaces?

This website has consumer reviews:

http://www.consumersearch.com/furnaces

Here is information about Rheem furnaces:

http://www.rheem.com/products/heating_and_cooling/gas_furnaces/

Here is information about Trane furnaces:

http://www.trane.com/residential/products/furnaces

Here is information about Bryant furnaces:

http://www.bryant.com/products/furnaces/gas/index.shtml