Small softbox - clip the gel to the front face of the softbox.
Large softbox - go inside the softbox and use gaffer tape.
(My Elinchrom lights have tungsten modelling bulbs, and I sometimes cut donut-shaped (and sized) rings of gel that are held in place by the neck of the bulb - i.e the gel covers the flash tube, but not the modelling light. I do this with ND gels. But it looks like your lights don't have old school modelling bulbs, so this won't work.)