Basics

Essentially Chafing Fuel but without the can
So either in bulk, or in a small plastic bottle (akin to Lighter Fluid or tube (akin to toothpaste))
Main advantage is reduced spill risk and POTENTIALLY less risk of Boiling Fluid in more improvised/simple Spirit Burner setups
Downsides are cost, residue after burning (depending on what gelling agent/dyes and other additives are, and more difficult dispensing (Gel/Paste vs a Gas/Liquid etc)
Also can be used as a firestarter/“liquid tinder”, as a safer alternative to Liquid Lighter Fluid (or Gasoline/Diesel Blends, using just the former is notoriously hazardous due to the vapor cloud)

Gel Fuel