Can those who have not done samashrayanam perform Gaya shraddha? Will it yield fruits? Should it be done as a couple?

There is no relation between pancha-samaskaram and gaya shraddha. Gaya shraddha is a sAvakAsha shraddha – done at a time of our choosing. Samashrayanam must be performed without delay right after upanayanam; it is better to perform Gaya shraddha after samashrayanam.
In Gaya shraddha, piNDadhAnam is important; in our tradition, piNDadhAnam is done during the pArvaNa shraddha; a householder (Grihastha) must perform the pArvaNa shraddha with his own agni; since aupAsanam is to be done, it is better to perform Gaya shraddha as a couple.