Hi Ravi,
we can check the Proforma Invoice in GTS under Customs Declaration.
go to GTS Cockpit>Customs Management>Export>Enter and Process Customs Declaration. there you need to use 'Process Customs Declaration' and press F4. here you enter the Proforma Invoice Number in the Document Number field and press enter.
if the Proforma Invoice is transferred to GTS then as per the Configuration settings maintained relevant Customs Declaration to be created automatically. so, if your Proforma Invoice is transferred to GTS , then you can see the Customs Declaration generated . if no Customs Declaration exists it shows message as 'No values for the selection'. then you can check in the Transaction - SLG1, which will specify the issue for not transferring the Proforma Invoice to GTS from ECC.
i hope you are clear now.
Regards,
Srinivas Yellampalli