User blog comment:Winturwulf/Does anybody know what this arm armor is?/@comment-6298283-20140302184044

Its really hard to tell. Honestly, it could just be gear designed by the artist of the illustration.

Also, Grette may simply be wearing arm gear that was never made available for the player to use...

If I had to guess, its one of the following (or similar to):

Bonds of the Dragon

Scholar's Bangle

or

Alchemickal Bangles