Gospel harmony
My experience with gospel is that it's basically harmonizing a melody with block triads (simply double the soparno part if you have a bass part). Mostly parallel motion: contrary motion should be reserved for special effects; same with unison/octave doubling. It's a deceptively simply style, harmonically speaking.
For example, say you have 2 bars of a C chord in the key of C, and say your melody walks up the scale C-D-E-F-G. I'd harmonize it with the following triads: Cmaj-Dmin-Cmaj-Dmin-Cmaj. That means, while the sopranos sing C-D-E-F-G, the altos would sing G-A-C-D-E, and the tenors would sing E-F-G-A-C (basses would double the sopranos an octave below). This should work whether your C chord is Cmaj, Cmaj7, or C7.
Much of the style, of course, is in the articulation. Getting the choir to sing a part with the same elaborate phrasing (while belting it out), I think, is where most of the effect comes from: that they happen to be singing different notes just magnifies it. Concentrate on the phrasing, and the harmonization generally takes care of itself.
4+ part harmonies and you're not really talking gospel anymore; that's jazz harmony (like Take 6 or Manhattan Transfer). At that point, the harmonizations tend to become exponentially more complex