What is the easiest way to get the Gaster door in Undertale?

So when I'm watching YouTube I see a door at Waterfall and I want to unlock the way to get the door. I have tried everything and really need help with this!


Solution 1:

You have to enter the game's files, and there should be a .txt file, that contains that character's "FUN Value". Change this value to 66, and you should have a decent chance of finding the door at the long corridor at Waterfall, just after Sans's Outpost, which follows the Papyrus battle.