The best replacement for gouache is watercolor.

The best alternatives to gouache are rated as following:
★★★★★  watercolor
★★  acrylic paint
★★  oil
★★  tempera
★  opaque watercolor
★  casein
★  poster paint
★  oil paint
  egg tempera
  acrylics difference
  watercolour
  holbein
  tempera paint
  acrylics
  water color

Your alternative for gouache is not listed? Suggest!
Speaking about gouache, one probably shouldn't forget about pastel.

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



Google Trends

Scale is based on the average worldwide traffic of gouache in all years. Click on graph for more.
gouache
watercolor
acrylic paint