Consider the chord structure of the hymn. Look at the key signature to see what key you're playing in. Scan the hymn to see what type of chords progressions are used. A vast number of hymns rely heavily on a simple one-four-five chord progression with the occasional minor sixth or minor second chord thrown in.
Substitute a chord formation in your left hand for what is written in the bass clef. Generally the bass clef will contain a two-note version of a chord. In order to thicken the sound, try a three- to five-note chord in the left hand.
Create a steady rhythm by alternating notes in the bass. For instance, if you're playing a G chord in four-four time, you can play a G with your little finger followed by B, D and G (one octave higher than the root of the chord). If you rock back and forth with this combination, you can create a four-four or even a two-four rhythm. For three-four rhythms, play the bass note with your little finger on the first beat and the other three or four notes of the chord twice with your other fingers for the second and third beats.
Play extra notes in your right hand as you play the melody. For instance, if the song moves from G to B in the melody, throw an A in between the two notes as a passing note to help create the gospel feel. Play arpeggios to add interest as well. Instead of playing a high D by itself (in the key of G), play a rapid D-G-B-D arpeggio leading up to the high D. You can also play double octaves in the melody to continue the gospel sound.
Experiment, listen and practice. Go through hymns that you know and try different techniques (like the ones mentioned above) until your playing starts sounding like you want it to. There is no concrete formula. That's the beauty and essence of the gospel style: it involves improvisition, personal taste and creativity. Listen to other pianists to hear what techniques they are combining, and practice to recreate the sounds you like.