How do you get a uniform from the tunnel HeartGold?

You can't actually get a uniform from the tunnel in HeartGold. There's no item or event associated with that.

Here's what you might be thinking of:

* The "Tunnel Uniform" is a Trainer's custom outfit: The "Tunnel Uniform" is not an actual item in the game but rather a custom outfit you can create by selecting the "Tunnel" option when customizing your trainer's appearance. You can do this after choosing your starter Pokemon.

* The "Tunnel" is a part of the "Trainers' School" mini-game: This is a fun little mini-game accessible from the Goldenrod Department Store. You can earn badges and outfits there, but it's not related to the actual tunnel in the game.

Let me know if you have any other questions about HeartGold!