La Banquise has the best vegetarian (red wine) gravy of any place I've found in Montreal that offers vegetarian poutine. I'd even go as far as to say that the vegetarian gravy is better than their "normal" gravy. I've let anyone I've taken there try it (some serious carnivores) and they've all agreed that it's superior in every way.
They also recently started offering vegan poutine made with the vegetarian gravy and Daiya vegan cheese. I'm not a fan of Daiya, so I can't tell you if it's any good or not, but the gravy alone is worth the trip.
They're open 24 hrs (one of the few places I've found in MTL) but parking there is *terrible*. It's also right across the street from a police station so think twice about parking illegally.