Gouache and Watercolor on Drawing paper?

That would always depend on the specifics of the situation. The difference between papers intended for dry vs wet media is simple one of weight and sizing. Technically, there is nothing wrong applying wet media on a paper intended for dry media as long as the paper can withstand the weight of the paint and is not so absorbent that too much binder is leached from the paint. You should be able to assess this as soon as the paint is completely dry (do not move the work until it is dry especially if the paper is thin and/or very absorbent). Does the paint readily powder off when touched? Is the paper severely buckled and likely to tear? If not you are probably fine.  

​I agree, if the paper is sufficiently heavy and composed of high quality fiber content, there should be no technical problem with using it for wet media. In terms of appearance, however, transparent watercolors can look a bit lackluster on unsized paper. Also, the soft, easily embossed surface of some printmaking papers doesn’t stand up to the rigors of harder drawing media. A sensitive artist can leverage those properties, though, especially in mixed media where “sparkling” color might not necessarily be the desired effect. The soft, velvety look of watercolor on, say, Stonehenge paper might be perfect in combination with soft pastels and charcoal.