What additional information do you put in the KML and GPX files?

In KML, we don't add anything, because the format is not extensible, and it would lead to compatibility problems with other programs.

When using GPX, LoadMyTracks can also incorporate additional data as defined by Geocaching.com and some additional pieces that have been created by ClueTrust. The additional formats we've defined are designed to capture data that would otherwise be non-conformant (such as data points with no latitude and longitude) or additional data available only on some devices, such as Cadence, Laps, heart rate, and temperature. In the future, we hope these mechanisms are useful to other software companies doing diagnostic or interpretive data analysis.