Well, now I've listened to it and looked at the score while I was doing this and the F# only occurs in this one bar, I think. At any rate, most of the time the D chord is a minor (with F natural) or lacks the third. It contains a few other chords diatonic to Gm or B-flat major, which is the relative major of Gm, or put another way, has the same key signature. Sometimes it has more of a major feel but it keeps going back to Gm. This is typical; lots of songs do this. Most of the time, it alternates between two chords (Gm and D(m)7), giving it a kind of feeling of rocking back and forth rather than moving toward a goal and ends with a kind of an ambiguous sound: the fifth in the top voice (D) whereas a G would normally be expected for a decisive conclusion.