I guessed three of them before opening the link. Namely:
1) Donna Summer
2) Sylvia Robinson (did you know that Sylvia knowingly nicked big chunks of Cavern by Liquid Liquid when she co-wrote White Lines with Melle Mel? Liquid Liquid sued and won. White Lines is the better track though IMO).
3) Our Gillian

Now, now. While they chose a bad example of a track to credit her with (no proof she was integral to the creation of Blue Monday, and Bernard feels the need to emphasize it by taking over her keyboard whenever it's played live), she certainly deserves credit. Hasn't she contributed to 8 New Order LP's (Movement, PC&L, Low-Life, Brotherhood, Technique, Republic, Get Ready and Music Complete)? And written they're biggest "hit"? I know you've been dismissive of The Other Two's output (the 1st LP sounds mostly like tarted up demos) but I think The Other Two and You is quite good. Except for the two awkward tracks that open the LP (yes, the two singles), I still enjoy it.

I think she [Gillian] deserves it. Being part of one of the bands that expanded the boundaries of electronic dance music for a long time she should receive part of the credits too.

I read Sumner's autobiography over the holidays and he says that Blue Monday was primarily his and Steve's creation. He even says that there was a slightly different version of it at first, but they accidentally erased it and had to reprogram the entire thing by memory, haha. As for Sumner taking over Gillian's synths live, from what I'm hearing they are both doing a duet. She's playing one melody on the lower notes, then he joins in to play a different melody a few octaves higher.