Gissur played a major role in the period of civil war which is now known as Age of the Sturlungs: he fought alongside Kolbeinn the Young against the forces of Sturla Sighvatsson of the Sturlungar clan in the Battle of Örlygsstaðir in 1238 and led the force of men who murdered saga-writer Snorri Sturluson in 1241, at the behest of Haakon IV, King of Norway.
Gissur had a significant impact during the civil war period known as Age of the Sturlungs: he participated in battles against Sturla Sighvatsson's forces and orchestrated the murder of saga-writer Snorri Sturluson, both actions carried out at Haakon IV's behest.