Lol. Moose. They're all particularly enticing times to start this thread. I guess it all depends on how we're approaching this. Since I assume this is a romantic type thing, maybe it will be best to start leaving hints before Dean dies and the angels are brought into this. Just little things. That way the development of finding out Gabe is an archangel is Sam's journey of finding out just who he's gotten tangled up with.
Then again, after Hammer of the Gods would be easier if we want to go off-cannon since it opens up whole new possibilities. Geeze. Let's say....Trickster era. Maybe during the Groundhog Day episode? That's very Sam and Gabe focused. Don't you think?