Currently it always defaults to base results on "Chip" time when you open the timing screen. Of course if chip start times were not captured, then the software uses Gun time by default.
With the current version this is a setting that applies to all races pulled into the timing screen. With the next update, you'll be able to specify if you want results to default to Gun or Chip time for each race, and that setting will persist. To change between the two, just click on the icon I've circled in the screenshot below: