1. - Sealing the surface of the paper will keep the water with the paint instead of soaking into the paper (as Rob said above). It also means you can use a lesser quality of paper. It's a different kind of surface. One of my favorite threads on WetCanvas is this one by Maggie Latham about using Art Spectrum Colourfix as a surface for gouache (though her use of it had nothing to do with extending the drying time):
2. - I've seen mention of using Winsor & Newton Watercolour Blending Medium with gouache to extend the working time. I've never tried this, so don't know how it would affect the gouache-iness of the paint. (I just tried and failed to find the blog I got this from - sorry!).
etc. - A spritzer and a Stay-Wet palette, of course (file under "you-probably-already-know-this");
I have mostly M Graham gouache, and a couple tubes of Da Vinci; the M Graham starts and stays wetter longer, so the brand you use might make at least a little bit of difference;
A 3-lefts-to-turn-right idea - if you mix a little of a very wet watercolor like M Graham into some gouache on the palette, it might stay wet longer (check pigments and try to add like to like). It will probably affect the gouache-iness of the paint of course, and any extended drying time will probably be outweighed by time spent fiddling around with tubes.