PD's are indeed used in gallons for exchange. However it is not directly to gallon himself or that shop area. In the quest (in pioneer) you have to go down to the...open area I want to call it, where you see space outside and whatnot. Just direcly right of the ramp (when looking at the ramp going from bottom to top) is a side hallway in which a fonewm is stood, this is paganini's young one (hopkins? lol) if you have a PD in your inventory you will be able to access the trade for PDs.
Note that in order to pay for srank specials + adding spheres etc are also here (but require east and west tower completion with collecting data from terminals).
PD's are also a considered value in which the economy bases all trade with.
EDIT: OH, The actual quest. I canny read. I would check episode 2 if i were you. You are most likely looking at the shop list in episode 1 , and gallons is an ep2 quest.
