I have taken dried gouache (rock hard) out of the tubes and put it in small containers and added distilled water and had it soften.
I don't know if it works with all pigments. And none of mine would have been more than about 5 years old at the time.
My favorite containers were old film canisters...but sadly, those have gone by the wayside.
Another thing to know about gouache is that reconstituted gouache always seems easier to use than gouache that is mixed, right out of the tube.
The very first time I used gouache for writing with nibs, we were told to mix up our little pans of gouache, first thing in the morning.
We did not get around to using it until later in the afternoon and I wondered why we had mixed it up, and then just let it dry out.
The instructor explained that letting the gouache rest a bit makes it better for nibs.
So, maybe it's an old wives tale - but, I seem to like my reconstituted gouache better than freshly mixed gouache.
Jean