The original definition of grognard was related (supposedly) to their grumpy, grumbling demeanor. Although a lot of fans of older editions of the game have made an attempt to "reclaim" the word and make it something positive, as far as I'm concerned, it will always come with a connotation of being negative and complainy.
Clearly, in context, it has nothing to do with any regiment of Napolean's army, so to the folks who keep throwing that out, you're being deliberately obtuse. Not cool.