You may have heard that KDE Plasma Next won’t support anymore the old X11,Xembed-based systemtray icons.
(More information here)

Years ago, we developed a nicer, model/view based alternative in which is the shell that actually draws the systemtray icon, allowing better integration with the workspace, it’s a specification that is now shared between KDE and Ubuntu Unity.
All KDE applications use it already, Qt4/Qt5-only application will use it depending on a small patch (and soon Qt5 will do out of the box)

But also GTK has some options: until today I was aware only about the Ubuntu’s appindicator library, but I have just been contacted by the author of another neat library, that can be found here on GitHub.
It’s a very small, few dependencies GObject-based library that allows a GTK3 application to export and control a statusnotifier-based systemtray icon. I just tested it on KDE4 and Plasma Next and seems to work quite well.
So if you have a GTK application that is using a systemtray icon, and you would like the icon to be integrated in the next version of Plasma as well, now you have an option more (and of course, the author will be happy of any patch/bugreport/bugfix).