Hmm. Had assumed they were demons from the name and alignment. However not really clarified elsewhere...I suppose it begs the question...what would be the difference if it was just a chaotic evil fiend vs a demon (narrowly-defined)?
Not much. Apart from the "political element" that Devil and Demons hate each other but sometimes work with other fiends, there might be spells or other magics that have specific effects on creatures with the (demon) subtype which wouldn't affect a Gurgotch if it isn't an actual Demon.