Way to create acidic/HCl gel?
It turns out somebody already answered the question in a different post:


  Xanthan Gum can handle hydrochloric / muriatic acid (HCl) for a while,
  depending on how concentrated the acid is. Here is how I made an HCl
  gel to clean an old toilet that was heavily coated with lime scale...
  
  I first dissolved a level teaspoon of xanthan gum into 1/4 cup of
  rapidly stirring water. After a few hours of letting it sit, with
  occasional stirring, I had a very thick, viscous goo. To make the HCl
  gel, I mixed one part of this goo with 1.5 parts of 31% hydrocholoric
  acid, by volume. This gave a gel of a suitable viscosity to get the
  lime dissolved off the toilet. I don't know how long this gel would
  last before the HCl broke down the gum, but my gel lasted several
  hours, which was enough to get the toilet clean.