You must have missed it. HK-51's gear is supposed to scale and was a feature up until a couple weeks ago. Apparently it is a known issue and is to be resolved. Hopefully sooner than later, the legacy unlock isn't worth the price otherwise.
Thought i remember reading something about the gear scaling to character level, just couldn't remember where x)
Good to know its a known issue, as i didnt see it in the known issues thread when i checked yesterday.