The Set GP Percent button allows allows you to set the gross profit percentage and markup percentage for Open status order only.
This can be done at the line level (Lines tab) or at the order level .
You can only set the gross profit and markup percentages if the status of the order is 'Open'. If the order is 'In Progress' or 'Posted', that means the order has a linked shipment and/or invoice.
If you cannot see the gross profit field on the header of the sales order, you can right-click and choose Customise Layout.(General tab).