Gouache requires good sealing to prevent any contact with moisture even though it is less likely to fade than other media. Gouache paint will revert to its paint-like condition once it becomes wet. The easiest way to preserve your finished gouache works is to waterproof them. Using a little brush and an enamel varnish or linseed oil-based varnish to seal your creations is simple.
[image from ivebeenframedpdx.com]