Hi,
I think you cannot modify the gain.
Only if you make your on antenna.
For a receiver, if you increase the sensitivity you can increase the range.
For a transmitter, if you increase the gain of the last amplifier stage, you can also increase the range.
But if you have no access to the antenna, its gain is fixed and cannot being modify.
Except if you improve the match between the cable that feed the antenna and the antenna, the total gain increase but i suppose that it is already realised.
good work.