It's a weird jumpsuit/romper thing, which I can only presume was made that way game-wise because it was easier to animate in Crystal Tools (Nanamo first appeared in 1.0 after all) than something else (this is also at odds with this old piece of artwork showing the Syndicate and Nanamo is apparently wearing a fancy pink dress instead, so I don't know). Who knows how SE thinks?

Either way, it's not actual 'gear' available in the game but is specifically part of her character model, and so it's unique.