Hello!
Percy's mom marries Smelly Gabe to mask his demigod scent, at least for a time. All demigods have a scent, some stronger than others. In Percy's case, he is a son of Posideon, one of the "Big Three". Meaning, that normally, he is a beacon for monsters. The Smelly Gabe scent masks Percy's scent until he is experienced enough to fight and kill monsters.
Hope this helped!
Sorry that I am late.
