It's Merlin/Gwaine, and it's called Friday Night with G. E. McGregor. More specifically:
Pairing(s)/Character(s): Merlin/Gwaine (background Arthur/Vivian)
Summary: Merlin gets the opportunity to meet his favorite author, but G. E. McGregor is not at all what he expected.
Warnings (if any): Modern AU, age disparity, Merlin is 20, Gwaine is 30ish
Total word count: 7.5K
And like I said, it's wonderful. Go and read!