I've checked out several means of doing this and recommend a simple utility called Always on Top, at the following link: http://www.labnol.org/software/tutorials/keep-window-always-on-top/5213/, or download directly at http://www.box.net/shared/4ej5cqoekj.
After downloading and unzipping the file, run the executable, select the Plan G window to ensure it's "on top," and then press Ctrl+Space to make it stay on top. If you want to disable this function and restore the default window behavior, press the Ctrl+Space again. I discovered that after I'd activated the function and minimized the Plan G window, I was able to use Ctrl+Space to alternately pop it up and minimize it, with it staying "on top" when not minimized
After downloading and unzipping the file, run the executable, select the Plan G window to ensure it's "on top," and then press Ctrl+Space to make it stay on top. If you want to disable this function and restore the default window behavior, press the Ctrl+Space again. I discovered that after I'd activated the function and minimized the Plan G window, I was able to use Ctrl+Space to alternately pop it up and minimize it, with it staying "on top" when not minimized