Galls in North America are edible, IIRC (better check on that before you harvest them for food). They come under that class of food we tend to think of as "emergency". I ate gall's collected in the Amazon that were part of the (extremely varied) diet of the local indigenous folks. The galls that they selected were rather cheesy with a sharp tannic acid flavor. I assumed that the cheesiness was due to the insect and the tannin was ude to the plant. In the tropic rain forest tannins are common as compared to alkaloids. The theory is that in terms of bioenergetics, alkaloids are more expensive (they're proteinaceous, requiring nitrogen) while tannins are built of sugars and thus, simple bonds of mostly C, H & O. These require less ATP's to produce and so, are produced abundantly. The tannins are what give the Amazon tributaries their black, light-absorbing color.