You don't need to understand music theory to learn to sing and follow the notes in gospel music songbooks. Learning basic music reading fundamentals will be helpful. Purchase a beginner's guide to reading music notation at a music or bookstore. You can also find the information online (see Resources). To understand gospel sheet music well enough to read and sing along, focus on learning about music note names and time values. This is basic music reading, and it's not difficult to learn.
Gospel songs have a similar sound. The more you listen, the easier you will recognize how many of the same chord voicings and melody patterns are repeated. If you can find recordings to gospel sheet music, follow the sheet music as you listen to the song. This will condition your ears to hear the intervals between the notes on the printed page while training your eyes to see the intervals. Eventually, you will be able to approximate those intervals at a glance. Purchase gospel "fake" books at music and book stores. A "fake" book is a lead sheet that indicates chords, simple melody, and lyric arrangements of songs, and they typically have around 500 songs in them.
Some gospel sheet music is written with melody, chords and lyrics. More complex sheets are written in SATB or some combination. This stands for soprano, alto, tenor and bass. The notes will be stacked on top of one another on the music staff and usually indicated by the SATB letters.. Find out which singing part belongs to you and pay attention to that line. Ignore the others. Unless you sing high or low, you are probably an alto and maybe a tenor. Someone in the church choir can tell you.
Speaking of church choirs, give it a try. If you've got the voice for it, singing in the church choir can be a fun, rewarding experience. There's no better way to enhance your music-reading abilities than to sing along with the church choir.