Hi GinaK this is quite a good resource for galls http://oneminutebugs.com.au/gall-inducing-insects/
But the short answer is that wasps will eventually emerge from them and they won't do much to the plant if there is only a small number of them. A large number can stunt the growth of the plant and eventually kill it. Gall producing insects are actually used as biological controls for plants in some instances.