If the rent recieved is less than expected rent( i.e. fair rent in this question) because of vacancy then rent recieved is GAV but if Rent recieved is not less than expected rent due to vacancy then we take GAV as Expected rent
For Ex- Rent recieved= 1000pm and it is vacant for 5months
fair rent =800pm
then in this case Rent recieved=1000*7=7000
fair rent=800*12=9600
if there was no vacancy rent recieved would be
1000*12=12000 which is greater than Fair rent so in this case we will take
GAV=Rent recieved i.e. 7000