My favorite Sam/Gabriel AU is The Dog Walker Verse (http://archiveofourown.org/series/30072) by LadyDrace. She also has a D/s verse with Gabe as the Dom, but I'm not sure if that qualifies as not PWP. There's another verse called Buttons On A Coat by GreyMichaela with top!Sam, also D/s, that has a little more plot. I have two more for Dean/Gabriel: Lysanatt's Theory of Silence and Storms (AU, where Gabriel wins Dean in a cage match) and Dynamic Equilibrium, where Gabe takes Dean as a state-ordered sub. (AU where people are born into their d/s roles.) I guess this says something about what I like to read :-) Oh all of these are on AO3, I didn't link to the ones that are more p0rn.