Not trying to be a dick, but youtube.com. If you search for GSC OU you find lots of content in total. There are alot of narrations of tournament games from this website which often guarantee very high level gsc matches. Look out for videos titled SPL (Smogon Premier League), WCOP (World Cup of Pokemon) or Smogon Classic. There are a couple of channels you can look out for too, Callousnarrates was already mentioned, he has decent commentary, very high level matches and is quite easy to follow. His main tier is ADV but he has several gsc battle narrations from like 3 years ago till now. Kevin Cronin, Bodybag Cased and PDC are smogon youtubers that have narrated some interesting gsc tournament matches that you could check out. There are more but you should be able to find stuff on your own.
Kaisser's suggestion is really good too, SPL, Wcop and Smogon Classic all include gsc, if you follow these tournaments a bit you will be able to locate tons of good replays for yourself to watch. Those obviously don't have narration though.
For the future please don't start threads for smaller questions like that and keep them to the simple question simple answer thread. Thank you!
