A gym is a building or large room that usually contains special equipment where people go to do physical exercise.
Positive: Dreaming of working out in a gym could represent that you are exercising to become spiritually fit. It could also be a literal dream about your physical body. If you look very strong in your dream it could symbolize you have strong faith or a strong calling on your life.
Negative: To dream that you are in a gym but not using the equipment could symbolize that the tools you need to strengthen your spirit are around you, but you are not using them.
(Proverbs 31:17; 1 Timothy 4:8; Proverbs 24:5; 2 Chronicles 15:7; Romans 14:1).