I can't really give you any definitive numbers until our engineers test final production units, so the following is tentative and subject to change and you can't bust my chops if it changes... OK?
You can expect the Gotham to have marginally better extension... maybe 1 Hz lower f3 than the Fathom. Much like the difference between the f112 and f113 which is about 1 Hz as well.
You can expect the Gotham to have very similar output to twin Fathoms (within 1 dB). While the power per driver is lower (1900W vs 2500W, which corresponds to a 1.2 dB difference), the box volume and driver differences may make up for that to some extent.
The reason I am being vague is that the enclosure has undergone significant revisions in the last six months as it has been prepared for production and the amplifier has also undergone revisions to qualify it for FCC and UL/CSA. We simply haven't tested a production version yet and those are the numbers that really matter.
You know the line.... Specifications are subject to change without notice.