The lead sheet that I found shows...
Bb that creates the
Gm7 is not in the melody in a harmonically essential way and that
Bb anticipates the same tone in the following
C7 chord... and the root doesn't change and the duration is short, chromatic passing motion seems like a good description.
The basic root progression by descending fifths isn't altered.
It's almost like a harmonic 'two for one' deal! You get the same basic progression as both a chain of dominant 7th chords or the standard
This is the whole lead sheet for reference...
EDIT re. the comments on the original question about whether this progression is common.
This is the exact same progression that ends the sections of the song 'Cherokee.' Both songs use the progression to lead to the tonic chord.
Like @ggcg said, I have seen this type of change in other lead sheets. Certainly the sound is familiar to me.
You could analyze it as...
V7/V v7/V V7 I
II7 ii7 V7 I
...either way the dominant 7th changing to a minor 7th is happening to a secondary dominant while the true dominant chord resolves to the tonic in the conventional way.
There is another example of this in Chopin's 'Prelude in E Minor' which is very clear. In mm. 4-5 there is a E dominant 7th chord in root position which changes to an E minor 7th chord in root position (then there is a diminished 7th passing chord) leading to an A minor 7th chord in 2nd inversion. The dominant 7th to minor 7th chord change happens again in mm. 7-8 & 15-16. Since the chord voicings are notated in the score the smooth chromatic lines are totally obvious.
So, is this kind of progression common? uncommon? How about uncommonly good!