Possibly. If you have an iPhone or a 3G-enabled iPad, then you can start using GPS apps right away. Wifi-only iPad, and all iPod Touch owners, do not have this luxury. There are a few bluetooth GPS receivers that you can buy to connect to your iDevice - doing so will let your iDevice serve as a GPS provided the receiver is connected - but aside from that, there is no way to use GPS without jailbreaking. Those who have jailbroken their iDevice may wish to download BTStack GPS from the Cydia Store, which will let any bluetooth GPS receiver send GPS data to an iDevice. However, this app will take over the default bluetooth of the iDevice until it is closed, meaning that braille displays, keyboards, headsets, and other bluetooth devices will be disconnected while the bluetooth GPS receiver is being used.
There are many apps for GPS navigation to choose from (the AppleVis website lists many of them and describes their accessibility, or lack thereof). Which one you choose is entirely up to you, and remember that you can use more than one GPS app. All of these apps will install whether or not your device is GPS capable, so be sure you can access GPS data before paying for an app. All iDevices have an app called Maps on them by default, which you can use to be sure your device is receiving GPS data.