How To Merge .gpx data into one .gpx data file?

So say I want to make a driving time lapse video of me driving through the mountains. I have a G-metrix overlay showing the rising and falling elevation over time. But if a video clip ends and another begins, that graphic starts over. Say I want to combine two videos from two different days into one video, is it possible to merge the G-Metrix data together to create one thing? It combines the two, say, if I have the track overlay on my video but I wanted to use that for the elevation/time overlay. 

Check this video out for an example: https://youtu.be/ME_QkLfg3A4