Since there's a problem opening *.gbx files with Trackmania 2 I found a solution, I hope it's a temporary fix though and that Nadeo makes it work like it did in TMUF.

With this you can double-click on *.gbx files to open them directly in play mode in Trackmania 2. The idea came from Electron's post but this way you don't need to use regedit.

• 1. Open notepad
  
• 2. Write:
  Code:
  

  C:\Progra~2\ManiaPlanet\ManiaPlanet.exe /useexedir /singleinst /file=%1

• 2.1 Make sure you wrote the correct path to your ManiaPlanet.exe (Progra~2 is the same as \Program Files (x86)\ in my case), note that you can't use blank spaces in the file path and thus write \Progra~2\ instead.
  
• 3. File -> Save as: save it anywhere
  
• 3.1 Save your file as: yourfilename.cmd
  
• 4. Right click on any saved track: Open With -> Choose Default Program
  
• 4.1 Browse to where you saved the file and select the file you created (yourfilename.cmd)
  
• 5. Done! You can now double-click *.gbx files to get into play mode on that track in Trackmania 2 immediately.
  

Note on 2.:

MrA says:
you could put the path in quotes and therefore allow spaces

may also work.

Original post here.
/»ѕωέ« ichi

Great tutorial!

Since TM2 uses he same extension for tracks as TMF, does this break the launching of TMF tracks? If Windows only sees the last extension of the file, then this is the case. Just a word of caution to people who are still active in TMF