That's the older realbook, which is incorrect. 6th edition has the following for the 1st 4 bars:I'm fairly sure its V7/II- in the Realbook
| Bb6 G-7 | C-7 F7 | Bb6 G7 | C-7 F7 |
which jives with what I hear on the Bag's Groove recording and also fits the melody in measure 1.
So you're both right
Of course there are a ton of standard substitutions for rhythm changes, but usually not over the head.
Cheers,
Jeff
While G7 is a fairly common sub for the Gm7, most recordings of Changes that I've heard use a m7 rather than a dom on the second chord.