That's way too much work for me. I just use a Sharpy and neatly write it on the folder tab.
That's what I do. I use hanging files, with clear plastic tabs. The gel number gets written on the white, cardboard label, and a little piece of gel goes over it in the tab. That way, it's easy to see the color and the number. Since I have a fairly small selection of gels, the whole thing fits in a plastic file tote which can be carried to where it's needed.