Hi David,
The transmitter gain can be changed by modifying the Tx1 and Tx2 Attenuation fields from the FMCOMMS2/3/4 tab of the IIO scope.
To expose the Tx gains in the Simulink model just add the following 2 lines to the end of the 'ad9361.cfg' file:
channel = TX1_ATTENUATION,IN,out_altvoltage0_hardwaregain,
channel = TX2_ATTENUATION,IN,out_altvoltage1_hardwaregain,
Regards,
Andrei