How do you get in to the flyers torn Pokemon Gold?

It's impossible. You can't. It isn't even coded in the cartridge.

The house was meant to be used for the underground route like the one in R/B/Y but this was scrapped due to time constraints. Like the Pewter Museum and Safari Zone, the place was closed. The programmers simply find no use for it when the Player already could use "Fly", rendering the underground area useless. However, there is one from Vermilion to Cerulan.