Putting gel on your gel?? Hmmm...
Anyway, I believe that unit is a Bowens mount, so there are probably bowens compatible gel frames - I know there's a barndoor with gel holder. Though for my similar sized Speedo heads, I just fold a sheet of gel over it and tape it, since when I use gels on a head with reflector I usually have a grid in there.
But for gelling softboxes, that's a different story since there's no reflector (the speed ring goes where the reflector would be). If the softbox face is smaller than a standard gel sheet, just clip the gel to the softbox with clothespins or metal binder clips. For big softboxes, see if you can attach the gel to the inner diffuser which is usually small enough. If there's no inner baffle, you'll end up taping gel sheets together or buying big rolls of gel and attaching sheets to the front face or to a frame.
Since most of the time people are matching their strobes to tungsten, Rose Brand makes 1/2 CTO diffusion material, or at least they used to. I have some 6500k HMI softboxes (more for video really) and I'm going to experiment with dying ripstop to match 1/2 CTO-1/8 minus green and just have replacement front faces sewn up.
But if you want to do other than the usual daylight/tungsten correction stuff with soft light, and your soft source needs to be pretty big? My solution is usually gel the reflectors and shoot through diffusion frames vs. using softboxes. That way I can use a quarter sheet of gel vs. taping several sheets together.