What I know about "sukima" isn't that much, but I talked with some people and it looks like they didn't really know. So here: (please disregard this if you already knew... I... I didn't read the whole topic, sorry XD)
A gap youkai can be varied, however some most famous stories are they were originally a female human. Due to an accident, or a crime, she was trapped inside a solid surface that accidentally has a gap that allows her to breath. It could be a wall, a pile of rocks,... The girl experienced fear, hopelessness, trauma and many things more. Her mind is pretty fucked up after that thus force her into a youkai. Her purpose can be revenge or just pulling people inside the gaps, like how drown people tries to get help and unconsciously kill others with them.
In the old time, this youkai was created after a landslide, people heard things from the crevices and made her up. Urban versions had her murdered and shoved into a wall. All of them are the same at the point of the "sukima" is invincible. She can be anywhere as long as there is a gap. And well, everywhere has gaps, you can't deny that. Even if you tried to sleep, there is a gap betweet your body and the blanket. Event if you run into a complete empty room with no door, no window, or in a ball that has no gap, there is a gap between your fingers, your lips,... You can't escape from a sukima once she's got eye on you. You can't defend against her, that's the whole point and why sukima should be so scary.
Because of her original death, getting crushed and pressed inside those wall/rocks, sukima's body is pretty much deformed. And her mental state won't help making her look any better. I think that somehow explain the things inside Yukari's gaps.