I’ve been watching your question hoping someone had an answer. I’m not from NO and have never been but the sandwich has interested me too. There are 2 places I know of that have (had) it on the menu in NYC.

http://nymag.com/listings/restaurant/gumbo-cafe/

This was the best prospect but seems to be now closed.

http://www.rosariosmenu.com/mainmenue.html

This is a popular place in NYC’s financial district. I had one here but it seemed that the guys making the sandwiches have no clue in what they are making. I can’t make an assessment; I’ve never had a real one. You may be better off concocting your own.

Happy birthday to your Grand Father and good luck.