raagas,

You had a valid view about the composer not 'working hard' when he is using his earlier tune. While that is true, I guess you must approach 'gum sum' more as a jazz song that our film song. In the sense, in jazz music, as you know, everyone plays some standard song, not necessarily something they have composed. What matters there is how they improvise on that song. If you also approach 'gum sum' that way, you may enjoy it better. Ofcouse 'gum sum' differs from pure jazz in the sense that no improvisation is done on the main melody but still...