how to get waterfall in pokemon gold!?

It's in Ice Path. When you enter Ice Path going to Blackthorn City, there is a small patch of ice, then a large one. Get across the large one and there will be a choice to go up or down. Go down and there will be another patch of ice with a poke ball at the right end. This poke ball contains Waterfall.

Of course, I'm doing this from memory. I haven't played Gold in 5 years or more. The location is correct, but the placement may be a little off.

Ice Path.

You get it on Ice Path. I think you have to beat Clair in Blackthorn City first in order to use it.

You find it in the Ice Path, between Mahogany Town and Blackthorne City.

You need to navigate some ice and rocks to end up at it though. It's not difficult though.