Gemini12

According to the Millingdale website, "Gemini12" was their secret ingredient in Henrik's Surprise, an ice cream flavour exclusive to Henrik & Son in Pete's World; (PROSE: Our Flavours) however, it seemed to actually also be a password to part of the Cybus Industries website (also confirming the suspicion of "ChatGuest1" and a tip they received from another individual).

The password turned out to be usable on the Cybus Industries Staff Intranet accessible on a page with something to do with "family" on the Cybus website. In turn, a second password, "binary9", was given. (GAME: Cybus Spy)

Notably, Gemini was the alias used by Pete Tyler when leaking confidential material to groups opposed to Cybus Industries. (TV: Rise of the Cybermen, The Age of Steel)

Behind the scenes
While it seems apparent that the name "Gemini12" is connected to Pete Tyler given that his alias is Gemini, it is unclear if Gemini12 is really the name of the secret ingredient or that the proper name had been swapped out.