the gain is from the module. I have an old dish antenna from my satellite TV diameter 1meter
a module is that the receive module ? transmit module ?
what is your intended transmitter output ?
a 1 metre dish will have a ~ 14 - 15 dBi gain at 900MHz
you need to be more accurate in you descriptions so that misunderstandings dont happen
a 20 element yagi will give ~ 17dBi gain at 900 MHz
2 of them in phase will add another 3 dB of gain
cheers
Dave