Hi, is this gel a potassium supplement? If it is, then I would guess that it's okay, and not life threatening. I wouldn't be freaking over it, but I would still call the vet to see what they think.
Hi Louise and welcome. We are only members here so my advice would be to call your vet or ER vet with this question. Hopefully you've done this since you posted 4hrs ago. Good luck and please let us know how your cat is!