If your doorstep is made of brick, concrete or other material like that there is your reason why the snake was there. He or she was likey trying to simply get warm as snakes cannot regulate their own body temp. That being said in many places it is against the law to keep native species as pets, so you might want to check your local laws. People do keep gopher snakes as pets and you can even buy them from breeders online, but taking one out of the wild and bringing it in if temps and surroundsings are not the same can cause the animal to go into shock. If you do decide to keep the snake look up care sheets, they are fairly easy to find on many websites and also talk to an expiernced breeder who can tell you more about them.