The Gatekeeper (Pyronia tithonus) is quite similar to the Meadow Brown (Maniola jurtina).
However, the Meadow Brown has no orange on the upper side of the hindwing, or very little (female), while in the Gatekeeper the upper side of the hindwing is orange with a wide brown edge.
The two species can be difficult to distinguish with closed wings since the underwing markings are very similar.
However, the Gatekeeper tends to rest with its wings open, whereas the Meadow Brown usually rests with its wings closed.
The Gatekeeper is also smaller and has double pupils on its eyespots.

The Gatekeeper also looks a lot like the Southern Gatekeeper (Pyronia cecilia), which occurs in Southern Europe and North Africa, but unlike the Gatekeeper, the Southern Gatekeeper has marbled hind wings and no small white spots on the underside of the hindwings.

Finally, the Gatekeeper also resembles the Small Heath (Coenonympha pamphilus), but the latter is distinguished by its much smaller size and the hairy backs of the wings.

The male Gatekeeper has a dark patch on the upper side of the forewing that contains scent-producing scales known as the androconia.
This is most likely for courtship purposes.
Androconia have evolved through sexual selection for the purpose of releasing pheromones for attracting mates.
Little is known about how androconia actually function during courtship, and the chemical composition of the pheromones is unknown.

1st photo: A male. Boekel area, The Netherlands, 6 July 2018
2nd & 3rd photo: A female. Boekel area, The Netherlands, 6 July 2018
4th & 5th photo: A male. De Groote Peel National Park, The Netherlands, 11 July 2018