Some red marks that are light indentations are completely normal. These are fit marks and are non-irritating. Fit marks are marks that you can see on your own body from the seams in your own clothes. When you take these clothes off at night you will see lines all over your body, but they certainly aren't painful. If, however, the marks are deeper and look irritated, then you may need to loosen the waistband as they are an indication that the nappy may be over-tightened.
gNappies have been designed so that the waist does not have to be pulled tight to function well (which is what parents are used to doing with disposables). As long as the trim on the pouch is sitting flush against baby’s skin and there are no gaps, then you should have a good fit without over-tightening or engaging the elastic.