The current text for this achievement reads:
Obtain a complete set of Grand Company soldier armor consisting of a soldier's cap, a soldier's overcoat, a pair of soldier's gloves, and a pair of soldier's boots. *Speak to Jonathan with all four items equipped.
I bought the Adders set, went to the npc Jonathas, and choosing either of the two options available in his dialogue box did nothing. So yah. This doesn't work.