This is the way I've been gelling lights for years - I call em "hats". Robert, and his mate don't mention one the key things about working with gels though. You need the light to go through a known amount of gel. There is no standard for this, however, most photographers working with gels typically pin a gel sheet to a 7" reflector and then bounce that into something if they need a larger light source. So, the area (3.5^2*Pi) = 38.5 square inches is the "normal" or typical amount of gel the light goes through. If you gel an entire 3 foot square softbox for example, and pump the same light output through it, the colour will be more saturated. So, when making my hats (and really, you don't need a bendy ruler to measure the circumference of the dome - you can use... maths! Pi*diameter will give the circumference) I use a piece about the same area as for a 7" reflector. Actually, a bit more tbh, as I like saturated colours, and often end up doubling up the gels on the reflectors. The piece in the video looked a bit small - which will result in a fairly weak colours (this is not wrong as such - you may want those pastel shades - just something to be aware of).
He and his friend have also missed a trick in making these really practical in the studio. When I'm shooting, I like to work fast, not just to get stuff done, but also for the sake of the model who's on set while I'm making adjustments to the lights. I get faster as the day goes on and by the last set, I'm just abandoning stuff all over the place to shoot more variations on the theme, or tweak something or other. The studio typically looks like an explosion in a lighting warehouse when I've finished... I don't have time to fiddle about with elastic bands on-set. I make my hats beforehand by just wrapping a piece of gel of roughly the right area around the glass dome on my SF600s and then I tape the seam with Sellotape. Then I fold over the excess gel and tape that down too. I make sure it's a bit loose on the dome and then I can just re-use these hats without any fiddle at all - they just slide on. I'll then typically attach the modifier first, and then fit the gel hat. It's a really fast way of swapping colours in and out, and works with any modifier.
Just one caveat to all of this - the heat, on the gel, but more importantly on the tube/light. I use Rosco or Lee high temperature gels. I buy them in big sheets, usually from Stage Depot
https://www.stagedepot.co.uk/lighting/lighting-gel . Unlike the AD600, my SF600's have incandescent 150W modelling lamps. I turn these off when using gels, however I have been caught out once or twice, where the settings on the trigger have overridden the settings on the light and the lamp has come back on and cooked the gel. However, even after a solid hour at gas mark 4, once the dome cools again, the high temp gels un-stick. I'm also normally using the lights on half power or below - and you do need to be mindful of how much flash energy you're pushing through the gel, and the higher heat build up on the tube. Make sure you leave at least some of the ventilation on the glass domes uncovered, and don't pound the light at full power relentlessly
If I'm using a reflective umbrella (and I've been having a bit of an umbrella renaissance recently
) I will just use a gel sheet over the 7" reflector in the "normal" way.
Some results (Model in bra and pants set - no nudity but probably not 'safe for work')
https://www.flickr.com/photos/owenlloyd/40561352412/in/dateposted-public/
https://www.flickr.com/photos/owenlloyd/39688778865/in/dateposted-public/
https://www.flickr.com/photos/owenlloyd/26498534208/in/dateposted-public/