This is a powerful gradient tool written in Tcl/Tk by Dave Harris for Windows and Linux. It runs on both Windows and Linux.
Features:
Download (you need to right click save as, because browsers complain about 'http' links):
Download Dave Harris Gradient Tool
Note:
On windows, simply run the RUN.bat file. On linux, you can usually install Tcl/Tk packages through your system's package manager (e.g., apt-get).