In Pokemon emerald what does the game designer give you?

The game designer in Pokemon Emerald doesn't give you anything directly. However, there is a character in the game called Mr. Stone who is the game designer for the Pokemon games in the region.

When you speak to Mr. Stone in Slateport City, he will talk about how he made the game, but he doesn't give you any items or Pokemon. He is mainly there to add a bit of lore to the game.

So, while there isn't a direct "game designer" who gives you items, you can meet the game designer within the game world.