I was using my GSC 10 in a race, which failed and only recorded speed/distance intermittently.
I was wondering if the garmin still calculated GPS speed/distance and if I could somehow find out that data or convert the file
If would depend if you have auto-pause enabled. Data isn't recorded when the Edge is paused. But if auto-pause is not enabled GPS data is continuously recorded and can be used to replace the bad sensor data.
The most likely cause of intermittent sensor problems is poor magnet alignment. Check that the magnet is aligned with the mark on the speed arm and that the gap between the arm and magnet is 2 to 5 mm. A weak battery could also cause problems.