Yousician supports songs in the .gp3, .gp4 and .gp5 file formats for guitar and ukulele.
If you have a .gpx file then you’re most likely using Guitar Pro 6. In order to upload your song to Yousician, you need to export it as a .gp5 file. In Guitar Pro 6 go to File > Export > GP5…
Then you can upload your song to Yousician as described here.