Gwaine just happens to know where the only Iron deposit in Camelot is, and can identify sediment like a pro.  Like here let me get out my munsell book that hasn’t been invented yet and give you a detailed history of industry and an overview of the geology in the region based on this soil sample.

He just has all sorts of random knowledge that probably seems useless until suddenly your mentor’s been kidnapped and your only clue to his whereabouts is the dirt on the bottom of some guy’s shoe. 

so stop complaining about his endless chatter arthur you might learn something