Yes, I am aware that The Once and Future Queen comes before The Witchfinder. I accidentally posted the Witchfinder one first, and I apologize for any confusion!
Arthur knows that Guinevere doesn't exactly approve of him. It's subtle, and she's never outright told him that he's a prat the way Merlin does, but he can tell. Normally he wouldn't mind but he happens to like Gwen—she's intelligent, pure hearted and gives good advice, not to mention Morgana is extremely fond of her—and even if he didn't like her she's Merlin's best friend and they're very protective of each other.
Which is why, when he decides to compete in the tournament incognito, he decides it's the perfect opportunity to win Gwen over.
Things do not go as planned.
Merlin dances around the sidelines, wondering what exactly his best friend and boyfriend are playing at with their sideways looks and muttered comments, but he's smart enough not to say anything. Whatever they've got going on, they are welcome to work it out on their own.
And, little by little, Arthur sees that he's earning Gwen's respect.
It takes a lot of trial and error (where, exactly, did that idea to cook dinner come from?) but by the time the final day of the tournament is upon them, he's half convinced he's won her over.
When he lets William take the glory for the tournament, Merlin beams at him, one of those gleaming teeth, crinkled eye smiles that just might turn Arthur's stomach to mush, and he doesn't exactly object when Merlin kisses him, pride practically seeping out of his skin.
Gwen clears her throat, and the two of them break apart and turn. Arthur had forgotten that she was there, but now he's a little embarrassed. Crown Princes aren't supposed to be snogging people willy-nilly, and it's her best friend's throat he just stuck his tongue down.
But then Gwen winks at him, giving him a small smile, and just says, "Shall I leave you two alone then?"
And Arthur knows he's got her blessing. And, just maybe, he's gotten himself a new friend besides.