I'm not really sure why Gothitelle was even necessary in this case. Perish Trapper M-Gengar has been a nuisance since it was released and was a large part of the reason why it was banned from OU. Gothitelle was another infamous trapper with Shadow Tag but having them both on the same team seems a little redundant. Anyways "fair" is completely subjective and isn't actually quantifiable or measurable, and Mega Gengar is allowed in Battle Spot, so I can't really say it was a fair or unfair strategy. I don't think it is, but that's just my opinion.
The way that strategy sometimes works is Gengar uses Perish Song on the first turn and to keep it from taking too much damage, it is switched out immediately into Gothitelle. That way the opponent is still perish trapped and Gengar can clean up later if necessary. As you said, "fair" is subjective, but it is something that a lot of people do because "winning is winning." I've seen people take 4 self-destruct/explosion pokemon with protect and 2 coverage moves (usually fire and ground) and just spam it the entire time. I don't know if it's trolling or just an easy win, but people do it.