The misconception about merwaine is that Gwaine is some suave dude who's flirting a shit ton with Merlin, knows exactly what he wants and how he feels. No, no, no friend. Gwaine is a wreck. He's hopeless. He's on his knees.
This is the man who said "Everyone gets tired of me eventually." He's a jumpy cat! Scared of intimacy. He doesn't know how to handle Merlin. He'd do anything for Merlin but he's quiet about it. He just shows up when he's needed cause he's got a 6th sense for when His Merlin™ needs help. And he can't lose this. Can't lose Merlin. Can't fuck it up like everything else. So he's gotta be useful. He's gotta serve where he can.
Merlin is the one encouraging more and more out of the relationship, luring this fool into being loved and appreciated. Til one day he goes "I love you, you know?" It's a quiet thing. Simple. Easy. Sweet. And no. Gwaine does not know. You wanna know why? He's a WRECK. But he's not a complete idiot the man of his dreams just told him he loved him, boom they're together. They have fun and make jokes during sex. They've got a playful banter thing going on, they're gigglin like teenagers they're even more insufferable than before no one thought it could get worse and everyone thought they were already together.