The best replacement for gel medium is matte medium.

The best alternatives to gel medium are rated as following:
★★★★★  matte medium
★★★★★  mod podge
★★  gesso
★★  floating medium
★★  gloss medium
★  modeling paste
★  modge podge
  fluid medium
  matt medium

Your alternative for gel medium is not listed? Suggest!
Speaking about gel medium, one probably shouldn't forget about acrylic.

Other search terms related to gel medium and their ratings are:
★★★★★ acrylic
You have in mind a companion for gel medium and it is not yet here? Propose!



Google Trends

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gel medium
matte medium
mod podge