What has Bernard gained as John's guardian?
At least for a while (particularly in Chapter 11), Bernard has gained a great deal from being John's guardian. Basically, Bernard has gone from being an outcast to being something of a celebrity and an important person.
After John comes to London, everyone wants to see him. Because Bernard is his guardian, they have to go through Bernard to see John. This leads them to suck up to him in hopes that he will connect them to John. This means that women who would not have been attracted to him before are eager to sleep with him. It means that powerful men are interested in hanging out with him.
So Bernard gains status in his society by being John's guardian.