I wouldn't take it during menstruation. A friend of mine did it, because she simply forgot to stop. She said this menstruation was so extreme. Once I took it in the morning at the first day of my menstruation which came (a bit unexpected) in the evening. That time I felt very very tired and thought that it came from the GCG.
I think they know why they wrote on the bottle to stop GCG during the mens.