I would like to use some KDE Plasma Widgets but don't want to install KDE and don't want to lose my current Gnome setup.
Can't say I tried this myself, not even sure it still works, but here is the link anyways.
I hope it isn't that late for this answer, but for new users looking for a way to run plasma widgets on gnome, this is what worked for me:
Your plasma desktop will change the background and show the low side panel with KDE gadgets, which can be easily deleted if you don't wish to carry it up by right clicking on it and choose "Delete this panel".
My desktop looks like this:
After that, just right click on the plasma desktop and choose "Add widgets" from the menu in order to be able to add, install and/or manage your widgets.
NOTE: I am using emerald theme manager with the "tron legacy" theme installed in order to decorate my desktop.
I personally don't find these widgets useful but it looks awesome. I must warn you that you will need lots of RAM in order to manage both desktops at once, my 4GB DDR667 is doing a great work but it takes 75% of the RAM when both desktops are opened with just a few gadgets, nautilus and dolphin working, a web browser, chat and a terminal. Just for your consideration.