It's been announced that John Romero will be releasing 18 new levels to celebrate Doom's 25th anniversary. Will GOG get this for their users?

https://arstechnica.com/gaming/2018/12/john-romero-will-release-18-new-maps-for-dooms-25th-anniversary/

Seems unlikely, unless GOG and John work out a deal to distribute it for free (and yes, I know GOG has worked out things with others like including that Deus Ex mod, etc. - doesn't mean they can do the same thing with this).

Otherwise, with the exception of the physical boxed products, you can always download it via John's website when it becomes available to do so.