They're not acting as a gatekeeper in the strict sense of the word, because T-Mobile is allowing customers to opt-out of the zero-rating services and have the data count against their data buckets.

Most issues with gatekeepers are what they don't allow people to consume; in this case, they're not placing a restriction.

If people wish to stand their ground on principle, this particular carrier is giving them the option to do so by opting completely out of the program (at least according to the article).