I'm going off my hunch here, but I feel like the manga glossed over some important details the original light novel have done. I'm guessing that the gyaru was the girl the MC confessed to in middle school where he got traumatized and converted into the 2D girls lover MC we see in the beginning of the manga.
This manga portrays the gyaru as some kind of force of nature lol, like she almost has no characterization at all. Her actions seems like it was based on a whim. It's baffling
Edit: man I'm glad I was wrong, still those chapters were a struggle to read