Hi,
During GR, you are referring the PO / scheduling agreement number only, not schedule line number. So you cant find a link between schedukle line and the GR document number directly, as of my knowledge. You can find in EKET table against the schedule line the GR quantity, but the material document number is not available.
However, you can extract the dat with BW. Pls check the link below.
http://help.sap.com/saphelp_sm40/helpdata/en/69/063f3f2615e07fe10000000a114084/content.htm
Hope its helpful.
Regards,
AKPT