Ga is just the available gain. If the output power is V^2/Rout, and Rout is the output resistance plus the resistance of the load ( or next amplifier stage ) and the voltage delivered is Vout *Rd/Rout + Rd (where Rd is the load resistance), then
load power over output power is:
(vout * Rd/Rout + Rd)^2 /Rd
----------------------------
(vout^2/Rout + Rd)
= Rd/Rout + Rin
Effective gain is Ga(Rd/Rout + Rd)
In the special case where Rd = Rout, the Effective Gain is: Ga/2, and that is the maximum Effective gain possble.