easy using xslt-1.0
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">
<xsl:output method="text"/>
<xsl:template match="/">
<xsl:text>longitude,latitude,time,elevation
</xsl:text>
<xsl:for-each select="gpx/trk/trkseg/trkpt">
<xsl:value-of select="concat(@lon, ',', @lat, ',', time, ',', ele, '
')"/>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
it outputs
longitude,latitude,time,elevation
12.123456,12.123456,2013-09-16T12:01:23Z,
12.123456,12.123456,2013-09-16T12:01:28Z,627.5