If there is PO. connected to one item and it goes to "analyze" step for some reason but there is no GR. then (if the "Block deviation approval if GRs are missing" is set to YES) you can not approve the deviation.

But if there are more lines and one of them is not connected to any PO. but the invoice is in analyze step - you have the opportunity to "approve" the deviation although there is no GR.

If no PO. connected it means no GR - the rule above ("Block deviation approval if GRs are missing") should consider this also.