Gallén's special field was the Middle Ages, particularly the Dominican Order's history in the Nordic counties, but he also wrote about the modern history of war. His most noticed work is about the treaty of Nöteborg and the medieval eastern frontier of Finland Nöteborgsfreden och Finlands medeltida östgräns (1968).
One of Gallén's thesis is that he holds it as unlikely that the Swedes made a crusade to Finland 1249-50 from a political point of view, because of a serious power struggle with Norway at the time. The second crusade he dates to the years of 1238 or 1239, which is more likely because that the papal bulla of crusade stems from 1237.
- Finland i medeltidens Europa (1998)