Punishing Fat gears guy

How do i punish GR Like his shadow command grab?

if by punish you mean when you avoid it, it depends how you avoided it.

if you dodged it by:
jumping straight up, both modes can do jHK on the way down to punish,
jumping forward, both modes can do jMK since youll most likely jump over him.
jump backwards, both modes can get jHP, but only if you were really close to each other so it might whiff.
if you backdash it you can usually get a MP or HP in there and get a combo punish