It relies on SAP Note 1502434 which is highly relevant to this case.
2310051 - KBA: Syntax error in program /SAPSLL/SAPLOBJSSF_SERVICES_R3
1502434 - Global Trade Services: Plug-in for SAP GTS 10.0 (Prerequisite: SAP Notes 762383 and 964632 must be implemented)
But this note 1502434 only supports the SAP_APPL version up to 605 which is too old. Finally, notes 1947027(Temporary inbound delivery transferred to SAP GTS) works perfectly to fix that syntax error by updating the below object:
R3TR - CLAS - /SAPSLL/CL_EX_PI_MM0B_TRANSFER
You do not want temporary inbound deliveries to be transferred to SAP GTS.
SAP Global Trade Services
Reason and Prerequisites
There is a program error.
Change the source code of the programs as described in the advance corrections.
Hope this could help anyone facing the same problem.
Btw one query is why the version of this updated class doesn't reflect the notes corrections.