The option --gui-mode has been removed with the following option: https://github.com/prusa3d/Slic3r/commit/82bde51c2752656aed517d89356272de277408b4
The option --gui-mode has been removed with the following option:
https://github.com/prusa3d/Slic3r/commit/82bde51c2752656aed517d89356272de277408b4

As it causes issues to the Repetier Host application
https://github.com/prusa3d/Slic3r/issues/363

this option has been added with this commit for compatibility reasons.

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r