The site has some location based services, such as the nearby page, or the distances shown on a guys profile. For your convenience, can derive your location from GPS, either to set your current or home location or to search for guys in a different location.
If you're on a mobile device, you probably already allow your web browser to use your location, (you can test this by opening Google Maps, in your browser, (not the Google Maps app), and checking it knows where you are).
When you log into dudesnude, there is a 'select my location automatically' option. If you're on a phone, this is the best option. If you're on a desktop/laptop computer, the location will be obtained from a database your internet service provider maintains. Most (though not all!) internet providers set reasonably accurate locations. If you're on a desktop/laptop and not getting an accurate location, you will probably need to disable the 'set my location automatically' option, and set home location manually instead (see above).
If you're not getting any location from GPS, and not getting asked if we can use it, you may have denied dudesnude access to your location at some point in the past. The easiest way to fix that is to delete the dudesnude cookie in your browser.