The other issue is that GPS devices are not as accurate as you might think, depending on circumstances. I have been caught out in marathons aiming for a particular pace, hitting this, but then ending up with a longer time than expected - all because either I had run 26.6 rather than 26.2 miles, or at least that was the distance measured by my Garmin. As a result, I have learn't that you need to aim for a pace about 5 sec/mile faster on my Garmin that I am after if I calculate what the average pace is to achieve my target time over the race distance.
There may be some situations where a GPS struggles more than others. I have run 10K on the track and there is more error in the GPS reported distance than over a measured 10K race on the roads. This will at least in part explain a problem in using your GPS to judge you pace when on the track. On Wednesday night we ran 1600m efforts. It was interesting to see how much out my Garmin was. For example, I ran one of the 1600m efforts in 6:42. This means my pace was 6:44 min/mile to the nearest second. However my GPS showed the pace for that effort as 6:59. Across the three 1600m efforts, on average my Garmin showed my pace as 13 second/mile slower than it actually was.
So the moral of the story: GPS devices are aids, but they are not definitive. It is better to run by 'feel' aided by your GPS rather than the other way around. In generally don't rely on GPS devices on the track.