A very brief guide about how to unlock all routes in Gore Screaming Show, though they’re not particularly hard to get, except maybe for Mashiro’s route.
|Akane, Aoi and Kiika’s routes|
All those three paths are available since the very beginning, they don’t need any particular requirement to be unlocked.
To unlock Yamiko’s route all you have to is to finish Aoi and Akane’s paths with any ending and Kiika’s one by getting the ending 「海の向こう」 “Beyond the sea”.
Automatically unlocks after completing Yamiko’s route and seeing her 「青春の日の想い出」 “Youth memories” ending.
The one I consider the bonus path is unlocked by seeing Kiika’s 「悪夢の果て」 “The end of nightmares” ending and Yuka’s 「時果てる夢－ｐｒｏｍｉｓｅ－」 “When dreams end –promise-” ending.