A D and a D7 are sort of, kind of the same chord. A D7 is just a D with a C added. So they both work in the key of G. So in some cases, maybe depending on the melody, the added minor 7th might sound nicer than a straight triad.
Strictly speaking, I guess a V in Nashville Numbering terms in G really is a D, not a D7. So in the studio, if they really wanted a 7 chord, it would be indicated.
I have a feeling though that you are not dealing with a studio score so in that case, the Nashville Numbering system is just a convention. How it gets used in real life and outside of its original purposes doesn’t always remain within the confines of the convention (hence my first statement).