I've called gc.setAntialias(SWT.ON);
and it does nothing. According to that method, it should work.
The Javadoc states:
Sets the receiver's anti-aliasing value to the parameter, which must be one of SWT.DEFAULT, SWT.OFF or SWT.ON.
It's not working for me, and I'm painting on a simple Canvas.
The following worked for me in an app I built and my guesses at when/how you have to do this.
So I created a new GC, set the Antialias as you did and then drew what I needed to with that gc object. The key is attaching it to the shell you will draw in.
Other than that make sure you do this before you draw anything. If you have to call it afterwards maybe try calling a repaint or redraw of the entire space.
Sorry without more information I am not sure exactly what is wrong.