The website describing the GPX standard describes a way to validate your GPX file in windows, however it does not mention a way to do it in Linux. So heres how you can validate your files (one way of many, I’m sure). The program you use is called xmllint, and in ubuntu, the package that contains this is libxml2-utils …
xmllint --noout --schema http://www.topografix.com/GPX/1/0/gpx.xsd testfile.gpx
xmllint --noout --schema http://www.topografix.com/GPX/1/1/gpx.xsd testfile.gpx
Hope that saves you some time!