1. You need:
2 game boy color systems
2 Pokemon Gold, Silver, or Crystal carts with mystery gift unlocked.
you have 1 game in each game boy, then at the main menu (continue, new game, etc) select the mystery gift option for both. Follow the directions, and the mystery gift will have taken place
2. I believe it's the large building in Viridian City. Someone who's played recently would be able to confirm or deny that.
3. It's because not many people PLAY g/s/c anymore. They either play the new generation or Kanto remakes. Or they don't play Pokemon at all