Classes at Gymboree are for children btw. 6 months and 5 years.
http://www.gymboreedach.com/en/location-basel.html
As for enrollments until the end of September they are offering a special, which is:
for booking one of the classes, you and your child can take another one for free (there is "specialized" MUSIC and ART classes.)
Preschool Playgroup, Play -GYM, Mommy-Baby Fitness etc...