I use a Ground Control Pro and 2 x Ernie Ball VPJR. When the Ernie Ball is direct into the H9, works perfectly. When mapped through the GCP to send CC02 and the H9 "Set expression value" is set to 02, the H9 will not respond at all to the VPJR.
All other CC mappings from the GCP to the H9 are perfect. No lag, no issues. Only the Expression CC will not work. I tried set to many different CC values and matching the two devices.
GCP Exp 1 and 2 send messages are one in MIDI track software and with Vox Tonelab, Axe FX, M5 and Torpedo Cab pedals. Neither will affect any parameter on any preset. including Vintage Wah or Pitchflex. I have an upgraded H9 to H9 Max.
Hope this is of some help.