I believe this is the fic you were looking to rec:

The Pretzel Model (As Applied to Time and Sex) (http://community.livejournal.com/nataliejfics/3477.html#cutid1)
Summary: Little things told Gabriel it was time to make a memory long held close to come true. Also, the world as they knew it depended upon it.

Aaaand... I'll be leaving a comment to the actual entry soon with my own recs probably. LOL. Can't pass up the chance to rec some good Sam/Gabriel fic. :D