To see one, look in tall grass close to hedges, trees or scrub, or anywhere with Bramble flowers, which are one of its favourite nectar sources. The adults don’t fly for very long and most have gone by the end of August. In warm, dry summers they can be seen in greater numbers.
The gatekeeper (Pyronia tithonus) is also commonly known as the hedge brown, but over the last 300 years has been recorded as many other names including the Hedge Eye with Double Specks! Their modern name probably comes from their habit of fluttering in gateways and hedgerows, with the male making small territories.
The males and females look similar, but the males have a dark brown smudgy line on their forewings known as an androconica or ‘sex brand’. These patches are made up of special wing scales from which chemicals called pheromones are released to attract females. In many other species, the sex brand of the male can’t be seen.
For more info see https://butterfly-conservation.org/butterflies/gatekeeper