You question is little bit not clear to me.
Do you want to change the G/L account which is coming on G/L account field in account assignment tab?
Its coming as per the setting of OME9.
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It will find the GBB-VKA with regards to valuation class in OBYC settings.
Refer to document Automatic Account Determination for without material master.
If your query is something else then please elaborate.
