I would avoid to use gouache for painting miniatures, metal or plastic.
The agglutinative of gouache requires an absorbing surface otherwise when dried produces surface craquelures, even applyed on cardboard or thick paper if it's not propely used can produce this effect, it's a difficult technique, very rewarding in terms of color richness, but really hard to master.
Mix with acrylics, as Svenn says, could provide and stronger agglutinative and avoid craquelures, but that will really depend on the composition of both, on the kind of agglutinative used on each.
I made a test many years ago and was a complete failure, at first moment seemed ok, but after some weeks on the flat surfaces (cloack, swords, etc) the paint started to crack. I left the miniature unvarnished, maybe a good and thick layer of varnish would help.
The good thing is water is enough to strip gouache.