Hi Dan,
We haven't tried this yet, and we don't have a pre-built solution that we can reference, but it should be doable.
PX4 on VOXL2 is setup mainly to use the DSP UART right now, which are setup for an ESC and a GNSS (GPS) unit. In theory, the gimbal driver would just need to use one of these ports, but I have a hunch there's some coding involved, and it would take over one of the two UART ports above which might not be OK for you.
We aren't doing this yet, but there are UARTs on the applications processor side that could be used via an add-on board for this use case likely.
So, to sum it up, doable, but not out of the box from our docs at this point.