Best Alternative Tubeless Tape to Gorilla Tape?

Your plan is correct. 21mm tubeless tape on a 19mm internal width rim is simply a mistake on their part. That won't create the contact needed between the bead and the tape. Hopefully they'll refund it.

On the bright side, you got some practice stretching the tape on, which might help you one-shot it next time.

What usually works is internal width plus 5-6mm. Going by outside width, i.e. holding up the roll of tape to the rim, you're looking for outside width plus 0-1mm on aluminum and minus maybe 1-2mm on carbon, since carbon sidewalls are usually thicker (that varies and I would always just use the internal width on a carbon rim, but I rarely do on al). It's a little bit of an experiment if you don't have a manufacturer telling you an exact number, but doing it this way will typically get you a width where the tape is just starting to kiss or ride up the inside of the sidewall, which is what you want. Plus 5-7mm works sometimes but not others; you can get in trouble with the tape wanting to curl over at the sidewall or get squished out of place by the tire if it's too wide.

If it's not clear, what's happening here is you want the tape to basically go from sidewall to sidewall, erring slightly on the wide side because you need the tire contact. The X factor is how much tape width gets eaten up by the rim well. Tubeless rim designs are mostly similar enough in this regard for the above rules of thumb to work. With practice one could probably learn to make corrections ahead of time based on seeing a flat versus upwards-angled rim shelf and other factors. But for the most part, +5-6mm gets you there. I'm somewhat copying this from WTB, who just say +5mm for all their rims and make rim tape to match these numbers. That works great for their rims but beyond that, one needs to know which direction to fudge things if an exact match isn't available.

One dynamic to this is that what shops really should be doing at this point is have something like the whole size run of bulk tapes from a couple different companies. For example, the shop I work at has almost every size of Stan's and WTB, except overlaps. Once you've got all the pluses and fats in there, it's like 12 or 15 rolls of tape. That buys high odds of being able to do any rim needed as a repair shop. But few if any shops would be able to have all those widths in retail size packs, and tubeless tape can't be resold from bulk without fouling it. So, walking in and knowing they'll have whatever size you need isn't a given. That said, the 24-25mm roll you need is common, so maybe it was out or someone was having a bad day.

In no case is there a need to get sealant involved until the tire is seated and holding air a while dry. Doing it all at once is fine when one has high confidence things are going to go smoothly, but that's almost entirely about saving time, not increasing efficacy. When things require experimentation, you should hold off until you've found a setup that works, then put your sealant in through the valve.

A final note is there are some rims out there with finishes that just don't play nice with Stan's-type tape. Some DT rims are the major example. DT makes a tape that works for these. We've successfully put it on at least one non-DT rim that wouldn't take Stans or WTB yellow tapes. The DT tape has a somewhat gummier, more flowy adhesive and that makes me suspect that Gorilla would also adhere, but I haven't tried it.