You pronounce it grognard, because in this context it's an American word and Americans pronounce all foreign words wrong.
(Sorry, but you all know it's true. )
"I choose option 5..."
"Three, sir!"
"... Option 3!"
And while we're on the subject of language, it's pronounced "row" to rhyme with "drow", not "row" to rhyme with "drow"! People need to get their language straight!