Galicia isn't deleted, Garcia merely got demoted to Duke of Galicia. Garcia never got the chance to rule as King of Galicia, his brother took his crown away before he had the chance, but Sancho did allow him to maintain his rule over the lands after he forced Garcia into recognizing him as his leige. Garcia being an independent king is a little silly and more than a little inaccurate.
If you want, you can make the Kingdom of Galicia claimable and give the crown to Sancho, so if you wanted to you could play as Garcia trying to regain his inheritance. Historically however, Alfonso revoked his duchy after he came to power. Garcia was just sort of weak and incompetent...