Move order appears to be very important here as the resulting positions can be very different.

I was focusing specifically on MNb's second move order with g5 being played.

I understand the former contributor, Neil McDonald, in his e4 book suggests that second move order.

It is at that point, 3.Nd2 Nf6 4.e5 Nfd7 5.c3 c5 6.Ngf3 Nc6 7.Bd3  when 7...g5 looks interesting. I can't find many long-play games on it though, although I have lost a rapidplay game on the white side of it.

I was wondering if anyone had any experience in that? Possible moves for white I understand are 8 0-0 and 8 g4, although 8 g4 might be answered by 8... h5

Assuming that you are thinking of 3.Nd2 Nf6 4.e5 Nfd7 5.c3 c5 the answer is simple: White does not have played the knight to f3 yet. 6.f4 and 6.Bd3 Nc6 7.Ne2 are more popular. After the gambit 6.Ngf3 Nc6 7.Bd3 (cxd4 8.cxd4 Qb6 "wins" a pawn) your proposal ...g5 has been tried a few times indeed.