Non Collection of GST and Penalties/punishment thereof

if you are registered in gst you have to charge unless composition registered
if you have not charged gst still you have to pay it to government by doing back calculation, it will be assumed you have charged inclusive gst. Not charging gst does not make you free from paying gst. it will cause penalty if you not pay it under gst payment rules.
further not charging gst in invoice will lead to contravention of sec 34 and you will be liable to penalty under chapter 6 for wrong invoice format