Black church music played a huge influence on jazz music. Black churchgoers learned harmonic styles which featured in church hymns. They would incorporate these styles into their own music and it is through this that jazz music relates to Gospel music. It is often said that jazz singers are also Gospel singers e.g. Aretha Franklin. Aretha started singing in her father's church and after that began a career a in jazz and R&B. Below is a video of Aretha Franklin singing a very jazzed up version of Amazing Grace. 

Most jazz singers are often compared to Gospel singers and for this reason, sing Gospel music. It is quite a popular tradition.