The wiki for reverse_gangbang is pretty vague. There's not much to distinguish exactly what it means to distinguish "having sex with" versus being the one sex is had with. A number of images tagged with reverse gangbang are a male (or gynomorph) with a number of females, but I'm not sure that necessarily warrants the "reverse" for example in post #804573 he's just laying back and being used by the women, no different than what's happening in post #2707755.

In this post 123easy lays out the definition that it is about who is taking pleasure from whom. There was no real dissent nor assent after that point, and I think it's a decent qualifier, this would mean that collaborative_* posts with sufficient characters collaborating would be a reverse gangbang, as would assembly_line posts; you could also quantify it on whether the focus character is active, in that case post #2410083 would be a reverse gangbang because she's going from penis to penis across the room. Alternately the definition could just be changed to "A group of characters all having sex with a male or gynomorph" I'm not such a fan of that one, but I'd rather get consensus before I start adding restrictions/definitions to the tag and changing things accordingly.