The common name Gopher Snake, in a broad sense, refers to the species
catenifer of the genus
Pituophis. There are six subspecies including Bull Snakes,
Pituophis catenifer sayi. All six subspecies are genetically compatible and can produce viable offspring. Additionally, Gopher Snakes (
Pituophis catenifer spp.), Pine Snakes (
Pituophis melanoleucus spp.), some Rat Snakes (
Pantherophis sp.), and some Kingsnakes/Milksnakes (
Lampropeltis sp.) have proven to be compatible and have produced viable offspring as well.
Here is a paper on the subject. Natural hybrids have been found in places where the ranges of certain species overlap, but they are rare. Most of these combos are known to exist only in captivity. For example, Jungle Corns (California King x Corn Snake) could not occur naturally as their ranges do not overlap. Personally I am not a fan of producing hybrids in captivity, but to each his own. However, I would be very excited to find a natural hybrid in the field. I saw a picture of a Timber Rattlesnake x Western Diamondback found in east Texas a while back, what a find that would be.