parameters.Add(new GPString("Expression", (AttQueryTextBox.Text)));
Currently, our GPString parameter cannot take special characters. This is a bug that we have yet to fix in our API.
If you can avoid using (",'), you can use GPString. If you need to use these, you can use GPMultiValue and hopefully that will be a workaround.
To be honest, I don't know how you would update your Geoprocessor parameter from GPString to GPMultiValue. But I do know that the GPString in GPMultiValue will support special characters.
The following help document might be useful:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00150000000n000000.htm