You can download all necessary schemas as a single ZIP file.
The main schema defining Payroll XML is PRL-GML20110324pxs.xsd. That schema imports and makes heavy use of a TransXML schema called TXL-GML20060501pxs.xsd. That schema includes another TransXML schema called LR-GML20060501pxs.xsd.
TransXML makes heavy use of GML. The specific schemas used are available here for your convenience. They are divided into three groups
Finally, some of the lower-level schemas make use of a few other general purpose schemas, schematron1-5.xsd and xml.xsd.