Yes, in order to participate in a quest, your mobile device must have GPS capabilities enabled. You must also have an active cellular internet connection with location enabled, and you must give CrypticQuests.com permission to access your device's location. For best GPS location accuracy you should have "High Accuracy" set on your phone.
Your active cellular connection must also have mobile data available, as this is the only way for you to interact with your quest.
A note about GPS devices:
GPS-enabled devices are typically accurate to within a 16-ft radius, but not all phones have the same level of accuracy. As a result, the exact location of your clue (Adventure Quests) or next point-of-interest (Discover Quests) will appear to vary, when in fact it is your position that is varying based on your phone’s ability to determine your current location. The accuracy depends entirely on your phone’s capabilities. This same phenomenon can be observed when using a Google map to find your current position. Your location appears to float and changes, because your phone is “listening” for signals from navigation satellites circling the earth. This is normal and does not indicate any problem with the Cryptic Quests system.