There is little known about the Battle of Gil'ead, except that Oromis and Glaedr were killed by Galbatorix, acting through Murtagh's body.
It can be assumed that the Elves conquered the city decisively and possibly with few casualties due to the sheer strength and magical power of the ancient race, as well as the support of Oromis and Glaedr. There is no mention of Galbatorix sending reinforcements or assistance with the exception of Murtagh and Thorn. Despite the fact that the elves won the battle some would consider it a loss by the death of Oromis and Glaedr.