That's a good question, but I assume that anti-trust regulators will decide what is "reasonable" partly based on the number of users of your messaging app.
Obviously there's a bit of a chicken-and-egg problem there, but if one of these gatekeepers has 100 million users, then it is hard to justify them writing and maintaining a load of extra code to interoperate with an app that has less than, say, 1 million users.
Perhaps in practice, though, the gatekeepers will be able to publish a simple API and say that all competitors who want to interoperate with them can do so, after signing a contract with their lawyers and exchanging public keys in person, and paying an administrative fee (to prevent spammers).
Obviously there's a bit of a chicken-and-egg problem there, but if one of these gatekeepers has 100 million users, then it is hard to justify them writing and maintaining a load of extra code to interoperate with an app that has less than, say, 1 million users.
Perhaps in practice, though, the gatekeepers will be able to publish a simple API and say that all competitors who want to interoperate with them can do so, after signing a contract with their lawyers and exchanging public keys in person, and paying an administrative fee (to prevent spammers).