Thanks all !
Menotu : Yes, I did reboot, without any success.
UncleV : Menu Editor doesn't have this Synaptic.
Neal : I can right-click Synaptic.desktop (in /usr/share/applications) and copy to the desktop, there it opens without root password. ( In a way it is good, I don't misuse Synaptic ), But it also leaves a desktop icon on E-17, which doesn't look nice.
Synaptic's desktop file, as root, showed "show in KDE" (or something like that, I forgot), it is the last line. I added ShowIn=Gnome to it and saved and rebooted.
No go !
Anyway, I can open it in console, it is good enough for me, I am too much of a noob to play around which may bork my system and I won't even know where to look and repair. Anyway, I have one more Gnome installed on another partition, if it comes to that.
Regards and thanks once again.