This how to assumes that you have already opened a gpx or kml file in GPX Viewer. If you haven't opened it yet here is How to open gpx or kml file.

Below are step-by-step instructions on how to visualize data on the track/route line.

1. Open left main menu in GPX Viewer.
2. Tap on Settings button.
3. In Settings tap on Track/route button.
4. Then scroll down and tap on Color line by data option.
5. In dialogue select from the drop-down menu under Data: caption for example Elevation. Note that the track/route you have opened from the gpx or kml file must contain Elevation data.
6. Then from drop down menu in dialog under Gradient mode: select option Dynamic (based on track stats). This option will adjust the color legend according to the track data.
7. Then tap on OK button.
8. Go back to the map screen and preview that the track/route line is colored by selected data. Also, note that at bottom of the map, there is shown legend showing which colors represent which data values.
9. When you tap on this legend you can change Data and Gradient mode options as in the Settings screen.

Below is a video showing the instructions above: