@jaderbavaresco - if you were recording with a camera without GPS as you mentioned, then in order for the .mp4 video and the .gpx to be in sync you would have needed to start each one at the exact same time. If there is an offset (like you started recording one a little bit earlier than the other) you’ll need to edit either the .mp4 file (for example to trim it) or the .gpx (with a text editor).
Once they are starting at the exact same time you should be able to upload the video file and the .gpx directly in the desktop uploader following these steps. Let me know if that isn’t working for you.