maybe you need to set the third parameter of GetFieldSelection, this is from Qlik online help
Returns a string with the current selections in a field. It is possible to query an alternate state.
ValueSep is the separator to be put between field values. The default is ', '.
Maxvalues is the maximum number of field values to be individually listed. When a larger number of values is selected the format 'x of y values' will be used instead. The default is 6.