I doubt it is a Gatekeeper. My book describes the hind wing as having an irregular white band with small eye spots on both sides. I only see spots on the outer side. Unfortunately the bottom corner of the wing is missing so we don't know if there is another spot there. However, what about a Small Heath? The small heath has small spots in S3, S4 and S5 as does this butterfly and the light spot between the inner and outer areas in S5. The colours don't look quite right but that can easily be the result of non-optimal photographic conditions.
Actually I was looking at my two books and not at the many images available on the web. There is in fact a lot of variability in the small post-discal spots on the hind wing as well as the size of the white area between the inner and outer bands. When the white area is small then it tends to be in S5, shall we say as in the photo.