I think it's funny it does not come with the GR55 synthesizer and you have to buy it apart.
There is no other way out, you need this cable.
I'm writing reviews on Vovox cables and after having one of them all other cables feel flimsy but this one from Roland goes beyond, for such an specific connector you wish they'd come up with a better result and presentation.
I like my cables neatly coiled over-under and tied, but this one comes folded in an 8 shape, the outer rubber feels cheap, the connector pins are very thin so I'd rather be extra careful when plugging in.
All in all it seems like Roland wants you to buy a lot of cables in the long run, and it's probably wise to have always a spare one.