This is quite a simple way to access your GMX online storage in nautilus, I found this trick on this forum post, so I'm posting it here in case it disappears.
1) In nautilus' location bar, type this in and press enter:
davs://storage-file-eu.gmx.com/ (or davs://storage-file-us.gmx.com if you live in the US).
2) Enter your GMX e-mail address and password when prompted:
3) And now your GMX storage appears in nautilus: