Can galls be formed from mutualistic relationships?
From what I know of ant-Acacia mutualisms, I've never heard of an ant gall in an Acacia. The link in your question also never mentions a gall. You've likely confused either domatia or Beltian bodies as a gall. Both differ greatly from galls in that domatia or Beltian bodies are created by the plant and not by the insect. 

Domatia are small plant-made chambers that house mutualistic insects (though are often invaded by non-mutualists).

Beltian bodies are a type of food body (or plant-made structures containing nutritional substances (often high in protein) that attract a mutualist species of ants in Acacia).

See here or here for more info about galls.

To answer your overarching question of whether there are mutualistic galls, I would have to say no. I do not know of any instances of which this is the case. Further, galls are inherently parasitic (not mutualistic). Even if the swollen cells do not interfere with normal plant function (e.g., as is common in Solidago spp.)1, the parasitic insect (or other arthropod) is still required to chew its way out of the gall. Last I checked, chewing on your host's tissues is never very mutualistic. 



 1: Crutsinger, G.M., Habenicht, M.N., Classen, A.T., Schweitzer, J.A. and Sanders, N.J., 2008. Galling by Rhopalomyia solidaginis alters Solidago altissima architecture and litter nutrient dynamics in an old-field ecosystem. Plant and soil, 303(1-2), pp.95-103.