I can't help but be disappointed with what WB did here. It seems like they lowballed the encode and cut down the sound in order to fit the 3D version of the film onto a BD-25 disc:
Disc Size: 23,393,149,583 bytes
Feature Size: 22,631,921,664 bytes
Video Bitrate: AVC: 16521 kbps; MVC: 10964 kbps
Audio: English / DTS-HD Master Audio / 5.1 / 48 kHz / 2369 kbps / 16-bit
(DTS Core: 5.1 / 48 kHz / 1509 kbps / 16-bit)
Yet The Hobbit: An Unexpected Journey 3D (and 2D, incidentally) gets to retain its Dolby Atmos 7.1 mix on the BD. Even though that film is split over two discs, notice how Part 1 is longer than Gravity is in its entirety:
Disc Size: 35,576,126,122 bytes
Feature Size: 35,108,708,352 bytes
Video Bitrate: AVC: 25974 kbps; MVC: 11993 kbps
Audio: English / DTS-HD Master Audio / 7.1 / 48 kHz / 5234 kbps / 24-bit
(DTS Core: 5.1 / 48 kHz / 1509 kbps / 24-bit)
The Hobbit 3D BD also includes several more dubs and subtitle options than the Gravity disc, and all of this with plenty of space left over on the BD-50.
I know encoding can be apples and oranges, but both of these films are relatively the same age, both used high resolution camera formats and Dolby Atmos audio, and both were delivered in the end as 2K DIs, according to IMDb.
I guess WB felt a surprise hit film, which is a critical darling and nominated (and really, a frontrunner) for a Best Picture Oscar is less deserving than a mediocre entry in a fantasy franchise.