Practice singing in a gospel style before the workshop. This is essential for demonstrating proper style and for instilling confidence in the group you're working with. Practice gospel singing by listening to and imitating some of the master soloists if this is new territory for you (see Resources).
Listen to each group's entire musical selection before offering criticism (time permitting). This will show them that you respect their efforts and are truly paying attention to their abilities and issues.
Work on vowel sounds. In gospel music, all vowel sounds should be as round as possible, as close to a mix between "ah" and "oh" as possible. The sound of these vowels should be thick and rich, resonating in the back of the throat, rather than thin or nasally (practice before the workshop so that you can demonstrate the difference in sounds).
Refine the group's consonant pronunciation. Gospel lyrics are often written in the proper dialect, but if not, explain that the "g" sound of words ending in "-ing" is dropped (for example, "mornin'" instead of "morning"). Some groups will add the "g" sounds back in unintentionally even if they're not written, so be on the lookout for this.
Make sure the group is singing with proper rhythmic emphasis. Gospel music, like jazz, should have heavy beat emphasis on beats two and four of each measure, but it's common for groups to feel beats 1 and 3 more heavily (since this is appropriate for most Western music). You may need to work with the accompanist as much as the choir on this issue.
Adjust tone and volume. Gospel music should generally be high energy and high volume. Tone should be rich and covered. Work with the group on proper posture and deep breathing.
Teach basic vocal improv. Though not all singers in a gospel choir will be singing solos at this workshop, they might all wish to learn how to do so. Gospel solos should almost all feature some vocal improv during held vowel notes (not scat singing). Let the group practice a bit of this all together; the individual solos may not sound great when combined, but the singers will feel more freedom to experiment.