I have contacted Telltale support and though not that helpful in actual providing an actual date, they say:
Thank you for contacting us. Currently we have run into delays with the GOG release of Batman: The Enemy Within. We are working on releasing this game on Steam as soon as possible. Unfortunately, we cannot go into detail about when it will be released. Please keep an eye on our support page for updates when this is released.
You can find our support page here: https://telltale.com/support/article/bat2-ep2-releasedate
Please let us know if you have any other questions or concerns.
And true enough, Gog is listed at that link as "Comming soon". For PC, it lists, as of this writing:
Telltale Store - PC: Coming Soon
Steam - PC: Available Now!
GoG - PC: Coming Soon
Windows 10 App Store: Available Now!