There are only three reasons why you would need an external GPS (such as the Garmin Glo).
1. Your tablet doesn't come with an internal GPS (which is the case with the Wifi-only iPads for instance. But all 3G iPads have an internal GPS).
2. You suffer from bad GPS reception in whatever location you mounted the tablet. This is mostly airframe dependent. Some people have never had a drop of signal no matter what they do, others report consistent drop of signal as soon as they move into the cockpit. Me, I know I should not put the iPad between the seats, but otherwise it's fine.
3. The Garmin Glo and similar supposedly have a higher update rate than an internal GPS, which leads to a more accurate track recording/position fixing. Plus, these external GPSs typically record the track you flew, independently and automatically. Which might be useful if you use an app on the tablet which doesn't do track recording.
My advice would be to try it out without an external GPS first, on a few flights. You can always get an external GPS later.
Last edited by BackPacker; 16th September 2013 at 10:47.