It was one of the question that arose in the comments here on FujIRumors, and we didn’t had a clear statement by Fujifilm until now.
Well, the Swiss site digitec.ch recorded its Photokina hands on the Fujifilm GFX with 2 Swiss Fuji Managers on youtube here (German), and at minute 5:35 of the video the Manager clearly says it will have 16-bit color depth. They also confirm what dpreview unveiled here: the GFX will be available in spring (and then they specify 1st quarter 2017. So this means end of March). At launch 3 lenses, followed by other 3 in winter 2017. The full specs will be officially released in early/mid 1st quarter.
Although the Swiss Fujifilm manager did sound very convinced about the 16-bit color depth, I’d still take it with a grain of salt. In fact, in the comments to his GFX first look (that I shared here on FR) Rico Pfirstinger said it will have 14-bit color depth.