Galls can be formed by insects, nematodes, fungus or other pathogens. Most do not cause serious damage to the plant (especially if the plant is mature and the galls are not numerous). More information about that here: Insect and mite galls

A photo can help me understand better what sort of growth/gall is present on your tree and any possible impact to overall tree health. 

You could contact a consulting arborist to come out and take a look at your hawthorn tree in person. You can find a local arborist using this site and the "Find an Arborist" tool: https://www.treesaregood.org/