What really is happening in the song is fairly simple--the melody, in the key of C, moves to a Bb--when something like this usually happens, we simply harmonize it with a C7 chord, or, if we want to be a bit more jazzy, a C9--The Gm chord can be substituted for a C9 for the simple reason that a C9 chord(C-E-G-Bb-D) is really a combination of a C (C-E-G) and a Gm (G-Bb-D)--The Bb is simply a major chord build on the melody note, which you can often do when the note falls on a one of the dominant beats in the measure(meaning, usually, the first or the third)--

When you go from a C to a C7, you are really moving from the key of C to the key of F(which is why we often move to the C7 when we are passing to an F chord)--of course, the Bb is the subdominant chord in the key of F, and the Bb note is on the dominant beat, so it is a fair choice to build a chord on it, as well--so it comes down to a matter of taste--