Well Gullahs are an old slave community that was isolated and held onto lots of African customs and such. It's about God though, I know that. I'm fairly certain it became Kumbayah because of how Come By Ya sounds in their vernacular. Gullah is a type of pigin English.
Kumbaya. ... Kum ba yah ("Come by Here") is a spiritual song first recorded in the 1920s. It became a standard campfire song in Scouting and summer camps and enjoyed broader popularity during the folk revival of the 1950s and 1960s.
I don't use the Wiki sites as authority but I thought it was good enough for this.