It's actually a visual novel series rather than a manga. (so unless you can read Japanese, scans won't do much)
The story is basically a sequel to one of the original Kamen Rider manga, but also tries to mesh in the continuities of some of the later Shouwa TV shows into it as well. Going on memory here, it follows some guy named Jun who was turned into a monster and his journey to becoming an actual Kamen Rider. (he's called Gaiaborg for most of the story and has a few forms, progressively going from monster-like to Rider-like) I think I remember Igadevil saying that Eve refers to the story while Gaia refers to the main character.