I very like KDE panel, so I installed it in Gnome. When I launched plasma-desktop, the gnome were removed and the plasma desktop appeared.
Is possible to use ONLY KDE panel in Gnome?

Currently, it isn't.

But it could be a nice thing to have. The problem is that I'm not sure anyone would be willing to code something like that (that is, most KDE devs use only KDE SC)

plasma-desktop always has the desktop background as well as the panel, but it would be possible to write a separate shell that runs only panels. kinda like plasmoidviewer (although that's a debugging tool).
it'd need to copy a fair bit of code from plasma-desktop, though... PanelView, PanelController, probably other things too..

When I've launched plasma-desktop in e17 (I'm not sure if you can get similar results in Gnome), I've noticed that the background and the panel open up in separate windows and I've been able to close them separately. You may be able to get away with launching plasma-desktop and only closing the background and leave the panel open. That said, though, I'm not sure if it is possible to set up a session to automatically do that for you (maybe with a script, but I'm not sure).

btw, Gnome wasn't removed when you launched plasma-desktop. It just launched on top of gnome's desktop, covering everything. You can always use gnome shortcuts to switch desktops to get back to the regular gnome desktop, or you can do alt+click-and-drag to move the plasma-desktop around (unless gnome did something to prevent such).