What are some side effects of inhaling gasoline fumes?

Bad. I assume you are talking about abusing fumes by inhaling, rather than incidentally inhaling a small amount of fumes when filling your gas tank. Inhalant abuse is EXTREMELY dangerous, with acute effects of feeling high, headache and passing out. You can become addicted after inhaling just a few times, and permanent brain damage can occur after 5-10 heavy inhalations. DON'T DO IT.