After reviewing some of the Workbench code and the MySQL documentation, it looks like column G is for Generated Columns .
In the source code in the mysql_table_editor_column_page file (line 132 at the time of writing):
model->model().append_check_column(MySQLTableColumnsListBE::IsGenerated, "G", EDITABLE);
Another code in the same area gives a layout corresponding to what you see in the visual for PC, NN, etc., which is available for viewing in the source.
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