Woah. Wasn't expecting this many replies. Yes, I do know what the portal to the gray building looks like, what I need help with is actually finding it in Guts World's massive map. (Thanks for the picture, though.)
To the guy above DMSwordmaster:
I'm aware that there's nothing to collect in the building. The reason I've been wanting to find it again is because I'm curious about the clown event and I've been wanting to experiment with it. To my knowledge the clown in the gray building is the only way to trigger the event.
And I have to agree with DMSwordmaster, there's a difference between being well hidden and hard to find because a map is so unnecessarily huge. If you had to interact with something like an obscure NPC for a portal to open, I would consider that well hidden. But when a map is excessively massive, it's not good designing, it's just an annoyance to the player. Wandering for an hour isn't fun, it's boring and monotonous, and nobody wants to do that. This is the same reason why I didn't like Lcd Dem very much - all the worlds are huge and you can't tell anything apart.
And I do in fact like exploration games, at least when it's done right. Why do you think I'm playing a Yume Nikki Fangame? :)