To give you context: this is the original line, with the red cluster that was noted wrong:
And this is my lead-sheet version of the 1st two bars with the corrected "Pink Panther sound" I asked for:
There is another thing he wrote in front the sheet music:
"Cm, of course, has 3 flats. But this way is easier to read for you."
So, the real key is probably Cm (###)
TC3 ...I didn't see a Gb in classical music for years! 😃 But it comes up in blues piano almost immediately -- we usually start in C major but include the so-called "blue" notes such as Eb and Gb. Fun stuff!
This is what I was thinking already in my OP post. Gb is used either in the key of Bb minor or Db Major, meaning 5 flats (or more). And 5 flats is really not beginner stuff (in classical music). But Blues (and maybe Jazz) are based on a different scale with 6 notes per octave. 3 of them chromatic, and the rest are 3-half steps. Music notation is really not meant for Blues.
btw: It is only this single note that has this Pink Panther like sound. I am not playing Pink Panther. (not yet)