"up in 'G'". anybody got any idea what that means and where it comes from?

There were 2 versions of the the song, and only the 2nd includes the verse you have quoted containing the phrase "up in G".

That is an important clue. "G" didn't exist at the time the original version of "Sidewalks in New York" was created in the 1890's.

"Sidewalks of New York" was reprised in the 1920's as we see from this history:

"The song was originally written in the 1890's, and was used as a Presidential campaign theme in the 1920's. Here are two versions of the lyrics. The first is as performed by Mel Torme. The second from the Digital Traditions lyrics database."

The first New York subway was opened in 1904 and, by 1920, the New York subway system was celebrated as an engineering marvel of the world.

"G" is a reference to the "G" train which was an express train going to the end of the line.

This is an elegant metaphor for the 'gay blades' who are long past their prime and can no longer 'kick the light fantastic' on the sidewalks of New York. You need to read the 2 lines which follow the lines you quoted to pick up this meaning.

Things have changed since those times,
Some are up in "G,"
Others, they are wand'rers,
But they all feel just like me;
They'd part with all they've got,
could they once more walk
With their best girl and have a twirl
on the sidewalks of New York


The celebrated New York subway was known as "the street under the sidewalks of New York", so it was fitting that the reprise of "Sidewalks of New York" in the 1920's would pay tribute to the subway in this way.