It's not gatekeeping unless you're using your own esoteric definition that's different from how everyone else uses it. Words get their meaning from how they're used in the wild, and dictionaries are descriptivist, not prescriptivist. Anyone is allowed to buy the book, so it's not considered gatekeeping, even if some people may not be able to.
If you define any obstacle to an activity as gatekeeping, then literally everything can be considered gatekeeping and the term loses all meaning. Some people can't read, so offering the books for free would still exclude them. If you include an audio book, some people still can't understand the language. And both charging and not charging money for products would simultaneously be gatekeeping under that definition, since not charging places more financial burden on the producer. Not everyone can afford to print books and give them away for free.
If you define any obstacle to an activity as gatekeeping, then literally everything can be considered gatekeeping and the term loses all meaning. Some people can't read, so offering the books for free would still exclude them. If you include an audio book, some people still can't understand the language. And both charging and not charging money for products would simultaneously be gatekeeping under that definition, since not charging places more financial burden on the producer. Not everyone can afford to print books and give them away for free.