You can automate volume in Garageband but I was pissed that I had to export to Logic Pro to automate panning. But I googled it and this guy has a workaround. Brilliant!
tl;dw: Duplicate the track, have one copy panned left, the other right, then just automate volume of each track.
Or export the said track to AUM, pan it there then bounce it back to GB!
GB has “open in” btw