In the key of G, there is one sharp, F#. G is the tonic, and D is the dominant chord-a fifth away.

A dominant seventh chord build with D in the root position would be D, F#, A and C. Hence dominant seventh.

The F# usally resolves as all common practice dominant sevenths does by leading up to G. The C resolves downward to B--the third of a Tonic G chord.

The C does not need to be flattened, as it naturally occurs in a G diatonic scale. However the interval of D to C is a minor seventh.

A seventh chord built on the tonic G would be G, B, D, F#-- It uses the interval of a major seventh. No diatonic tones were altered in the creation of this chord.

As far as being ok to play these chords, ask Eric Satie.