GPX is a somewhat loose format, so there may be variations that do not work with RaceRender, and non-standard or "extensions" data may not be picked up. Generally speaking, the file must begin with an <?xml> tag, and for GPX, each track point should look similar to the following example:
<trkpt lat="39.602613" lon="-105.020217">
<ele>1624.77</ele>
<time>2011-06-02T14:28:35Z</time>
<speed>31.2</speed>
</trkpt>
The time tag is always required. The elevation and speed tags are optional, but recommended.