Grails get view.gsp file output to a string variable

def formValueHtml = g.include(view: "pdf/customerAndInvoiceProductDetails.gsp", model: [
      invoiceProductInstance: invoiceProductInstance,
      invoiceInstance: invoiceInstance,
      customerInstance: customerInstance
]);

Where  customerAndInvoiceProductDetails.gsp is a view file.
And  invoiceProductInstance is data object.