Update:
I found a way to make it work on the X310 (and in a limited
way also for the B210)
If
I implement it as a bursty transmission (with tx_sob, tx_eob
and tx_time tag) I can calculate the time of the gain change
from tx_time and the sample rate and add it to the tx_command
tag.
This
only works for the X310.
For
the B210 the time tag doesn‘t work for some reason. But on a
bursty transmission the position of the tx_command tag works
to some extend (not on the exact sample asked for, but with
some delay).