Okay so here's some thoughts I didn't wanna have
So, ya know how Gwaines' got tears in his eyes here?
And, ya know how Gwaine would never think for one single second that Merlin would actually be abandoning the Knights in their hour of greatest need, let alone Arthur?
And, ya know how he accompanied Merlin to the caves without one question asked?
Here's tha thing: Gwaine knew about Merlin's magic and understood Merlin had some sort of plan; a plan which he couldn't speak to anyone about, including Gwaine. And knowing Merlin, this plan would ofc involve putting himself in the utmost danger. BUT Gwaine couldn't just leap in and say 'tell me your plan you magical git and let me help you' because of the honour involved in waiting for Merlin to tell him about his magic when he is ready.
Offering Merlin his sword? Gwaines' quip about 'you know to use the pointy end, right?' isn't the joke, the very giving of the sword is because Gwaine damn well knows Merlin doesn't need it. Also, this is Gwaine forfeiting his own only line of defence for his way back home; signalling his complete vulnerability being surrendered to Merlin.
But why is Gwaine crying? Is it because he believes Merlin won't survive?
It's because he knows he himself probably won't, and thus will never get the chance to have Merlin tell him about his magic, or tell Merlin how he feels for him.