You can make use of ME2L,ME2M,ME2N,ME2J.
In all the above reports enter plant,document type or document date as input field and then execute the report.
You will see the column Still to be delivered (both for Quantity and Value) thus you can get open PO.
Alternatively you can make use of Purchase Order history column in above mentioned report.
Sort the report in ascending order and PO against which no GR is made will be having Purchase Order history blank and those having history will be having special icon.