It's interesting because outside of her cover of 'I Want Your Love', Gaga doesn't really have a full-on disco song. As matter of fact, she rarely has dance songs completely committed to the original genre; it's most dance-pop with tinges of other things (disco, electronic, house, etc.).
But I guess my favorite disco-inspired track would have to be Fashion!, which is probably super unpopular but I love it so much The groove, the melody of the chorus, the Bowie 'Fashion' and Let's Dance references...it's just all so much fun and rich with high-octave camp My brand apparently