You may have a case against the gas station or security company if you can show that they were (or reasonably had to be) aware of the attack and still failed to act; in that case, their failure to take reasonable steps to protect you, a customer, from harm, could make them liable. If the attack were over so quickly that the guard could reasonably not have reacted in time, or if the attack were not visible (e.g. you were out of sight), they would probably not be liable. You could also potentially sue your attackers as well, if you know who they are. From what you write, it would be worthwhile consulting with a personal injury attorney to evaluate the strength of your case and what it might be worth. Good luck.