The SSC GD physical test has a timed race along with height and chest measurement standards.
Start running practice early — build stamina gradually with daily runs and increase distance over a few weeks.
Practise sprinting too, since the race is timed and you need both speed and endurance.
Keep your weight in a healthy range so the height-weight and chest measurements are comfortable.
Train in proper running shoes on a ground similar to the test track, and rest well in the final week.
Check the exact race distance and qualifying timings in the official SSC GD notification before you start training.