The outline symbols can only be enabled programmatically in a protected sheet using the EnableOutlining VBA property.
In other words, this EnableOutlining property doesn’t exist in the XLSX file, MS Excel doesn’t write this information in the file. Because of that, GemBox.Spreadsheet is unable to provide support for it.
The only way to achieve this would be to have an XLSM file with that VBA code and then use that file as your input template which you would load with GemBox.Spreadsheet, fill out with your data and save to output XLSM file. In that case, the output XLSM file will contain that VBA code because GemBox.Spreadsheet preserves macros.