When you select a sub-style, the backing instruments are pre-set. The Jazz pack>11 Gypsy jazz uses a guitar instead of piano (the mixer section will still say piano, but in this case the volume changes the guitar volume.)
Some other sub-styles use different bass sounds, different guitars, accordion etc. (all are currently locked into that sub-style.)
(ref: iReal book iOS v3.0.4, Mac v1.1)