how can i get the gp skin?

any game where youre matched with teammates i guess. just play a normal with him.

the only thing i wonder is how we get confirmation that we have it.

You can get it if you have played him at all in CO op v AI, Normal Blind, Normal Draft, Teambuilder, Ranked Draft, or I think even ARAM or Dominion since the rework came out and before the 10th. I have played him, but have not gotten any confirmation that I got the skin, so I don't think they are announcing that you got the skin or anything, they'll just give it too you on August 10th