What about a boot/seal on the bottom of the garage door to take up that gap better than it's currently doing? Something like this:
http://www.northshorecommercialdoor.com/ulrugadobowe.html?gclid=CjwKEAjwoLfHBRD_jLW93remyAQSJABIygGptio8PJX2v-RYv3FXpRmRUbqhlIhgmAWYYiX1luOmPRoCmarw_wcB