Hi!
I'll try to be as unbiased as possible...but I believe wild snakes should be left in their native habitat. If too many are taken out, the "pest control" balance will be upturned. Not to mention, that if more and more people take snakes out, we may face extinction of our native snakes.
However, now that you've taken it out...you have two choices. If you're quick, you can put it back where you found it. I've been reading that relocated snakes don't do well so you'll have to put it back where you found it. Your other choice is to keep it, as you've suggested.
> 1)reasons to keep it or let it go.
I've given you reasons above on why to let it go...if you wish to keep it, it is entirely up to you. My first snake was a native gopher and from there, I progress to other snakes. It was a hardy "practice" snake for me. Years later, I released it. But, I still wonder if I did the snake a disservice.
> 2)what to feed it.
A 4 ft or larger gopher snake can eat anywhere from large mice to medium sized rats. It's better to give him a few mice than a large meal, especially in the beginning to avoid regurgitation.
Snakes usually need a week or two to adjust to their new environment before they will feel comfortable enough to eat. Don't try to feed him/her for a while...snakes need to feel they are safe because eating leaves them defenseless.
3)how often to feed it.
Some people believe on feeding on a schedule. I do not. Because in the wild, food is found at random. I wait for my snakes to "tell" me they are hungry, by attentiveness and activity level and that changes with the seasons. If you wish to feed on a schedule, which entirely your choice, feeding the snake as much as it wants at a time (probably 3 large mice), every 8 to 14 days is more than enough. Some snakes like more...some less.
4)cage and bedding.
He should have a terrarium at least half as long as his length. The gopher will probably get to be about 5-6 feet long. For substrate, anything large like bark is good. Don't use the fine stuff because pieces can get caught in the animal's mouth and cause mouth rot. Of course, newspaper is fine too, if you don't want to spend the money on the bedding. The water dish should be large enough that the snake can get in and soak, if he so chooses. He will also need a piece of drift wood or some other piece of wood, so that he can use it to get his shed started.
5)where/how to check for internal parisites.
The only way you can check for internal parasites is to take a stool sample to the vet. Sometimes snakes have round worms and are visible in the fecal matter but if you're concerned, you'll have to find a vet check him out.
6)and other things to be aware of.
For digestion, depending on the temperature where you are, he will need a heat source. I would recommend an under tank heater.
You should keep him out of the direct line of air conditioners and do not put him in the direct sun, like near a window.
Good luck!