I was on Amazon Japan and I saw GXP 15 was the bestselling title under the Fujimi Fantasia Bunko label. It's even outselling Rokudenashi Majutsu Koushi to Akashic Records which has an anime airing right now, so I was wondering how well they sell.
(https://s11.postimg.org/e1dli2jf7/Untitled.png)
If I recall right the physical copies generally sell out at major book stores, not sure about online.