Receiver Antenna Gain
For an antenna that can transmit an EM wave it's receive gain is exactly the same as its transmit gain. This is down the antenna reciprocity: -


  Reciprocity is one of the most useful (and fortunate) property of
  antennas. Reciprocity states that the receive and transmit properties
  of an antenna are identical. Hence, antennas do not have distinct
  transmit and receive radiation patterns - if you know the radiation
  pattern in the transmit mode then you also know the pattern in the
  receive mode.


Quote taken from here.

Be aware that this is for the transmission and reception of an EM wave and not, for instance for a medium wave ferrite rod antenna - it can only intercept the "M" part of the EM wave when used as a receiver and it does it very well but, it makes a poor transmitter because it cannot generate a suitable "E" value due to it's small size.