I think I would definitely have if(!gs.nil(grTable.caste_field)){}

I think I would also do grTable.other_caste = grTable.caste_field.value;

In essence I would debug this by adding an additional criteria restricting the query to a record I know should be updated and outputting the value of other_caste and caste_field.

My personal preference would be to put this in a fix script, encapsulate the logic in a function containing a try-catch block with appropriate error handling and logging, will probably save you some time in the long run.

Hope this helps!

Rich