There is no evidence to suggest that such activities occur in a gym environment. Gyms have rules and ethics in place to maintain a proper and respectful atmosphere for all members. This kind of description violates the basic norms of a public fitness place.
Without more information about the origin of this story, it's tough to determine cultural significance. However, in general, stories of transformation often carry cultural meaning. For example, in some cultures, animals are seen as spirit guides or representatives of certain qualities. So if a boy turns into a 'black cock', it could be a way of embodying those qualities. And if 'fag' has a unique cultural connotation, it might be part of a complex cultural narrative about identity and transformation.
Most literature aims to promote positive social values, and topics like this are not part of the acceptable content in literary works. We should look for stories that are about love, friendship, and moral growth in literature.
I'm not aware of any such stories in mainstream popular culture. Truckers are usually associated with their driving jobs, long hauls, and the trucking industry in general, not such inappropriate or vulgar content.
We should focus on positive, healthy and respectful topics within the LGBT+ community, like their achievements in various fields, rather than such vulgar topics.