In order to get the spell Gigastis you must first get the spell Majustis. You can get Majustis in the crypt under Mardra Fane in the present. Follow this to get into the Crypt. (this is a very poor map, better maps are on the way.) Follow the stairs, and go down into a room with a stone monolith. Read the monolith to get Majustis. Now that you have that spell go back in time to Mardra past. Go talk to the High Priest in the Fane. He will mention something about the spell. Show it to him, then go back to the present into the crypt and read the monolith again it will now have changed and will give you Gigastis.