How to improve my Ground contact time balance?

I can not help you here in specific (what side needs the work?!), but I can say strength training of both legs will be helpful in creating a strong base that allows for good balance and confidence in strides.  

Try to focus on single leg activities to help uncover weaknesses and imbalance.  Single leg calf raises (toe in and toe out as well ... "three-way calf raise"), can do leg press/extension/curls all one leg at a time.  One leg at a time lunge type stuff ... one leg glute bridge ... one leg side leg raises.  

If you do calf raises or squats with both legs at a time... it is very easy to favor one side (without knowing/noticiing it).  So with your goal of GCT balance... single leg work will be great!

For me, I know my left leg is 'weaker' ... so i purposefully ONLY do calf raises on that leg some days (during curl recovery time...etc).

I really cannot say anything about the accuracy, except that placement of the pod (off-center vs center) can have an effect. But if your easy run is more or less balanced, that sounds ok. My recommendation would be to do leg strength training in general. If you concentrate on one leg only, you'll risk just causing more imbalance to your running form.

Throw away the running pod and start to relax. Nothing is wrong with you.

Well, everyone can do what they want, of course, but seeing ground contact imbalance while running fast has helped me a couple of times to notice signs of muscle injury in time to do something about it. So I wouldn't be so fast in throwing absolutes.

Thanks for the reactions. Maybe some more information. For some months I had pain in the back of my knee (knee cavity). I noticed that I had more pain after I did some interval training.

My fysio said that this was because of injury in my hamstring and that it was stiff. After some treatments and many stretching of my hamstring it was a little better but not totaly gone. So now I was looking at my balance.

I'm already doing leg exercises for more strength because I indeed feel that my left leg is a little less strong. 

You can do your own gait analysis. Video yourself running - back view, side view and front view. Use slow motion mode to record, most phones have that feature. You can do that on treadmill or outdoors. Any imbalance should be easily visible on the videos.