García Sánchez inherited the crown of Pamplona after the death of his father Sancho III in 1035, bypassing the late king's eldest son Ramiro, who was illegitimate.
In the year 1037 he joins his brother Ferdinand, the nominal Count of Castile, in a battle against the Kingdom of León that took place near the river Pisuerga and that came to be known as Battle of Tamarón.