In Handloader #73 (May-June 1978) Bob Hagel published a lengthy article on his experiments with the.30 Gibbs.
His summary sentence:
"To sum up the performance of the .30 Gibbs cartridge, it will produce about 50 fps more than the .30-06 with the same barrel length when both are loaded to approximately the same pressure levels."
The key phrase is "same pressure levels". Hagel has a reputation for tipping the powder bottle.
An earlier sentence is significant: "... with the best powders the .30 Gibbs will kick the 180-grain bullet along at between 2,875 to nearly 2,925 fps. This is really quite impressive velocity, and a great deal of improvement over the 2,700 fps considered average for the venerable old .30-06. But when you load the .30-06 to similar pressure, the best powders will show velocities running from 2,825 to 2,900 fps also."
Hagel wrote that the .30 Gibbs has about 5.8 grains more capacity than an unaltered 30-06 case with a 180-grain Hornady bullet seated. He didn't specify the make of case. Nosler manual #8 shows a capacity of 63.9 grains of water with an unspecified 180-grain Nosler bullet seated. Adding 5.8 grains to this shows the Gibbs cartridge with about 9 percent greater capacity. According to the Barsness rule of thumb, velocity at the same pressures should be about a fourth of 9 percent, or 2.25 percent greater for the Gibbs cartridge.
With a 30-06 velocity of 2700 fps, at the same pressure the 30 Gibbs will produce 2760 fps. If you can get 2850 fps from your 30-06, a 2.25 percent increase will yield about 64 fps greater velocity, or 2915 fps.
[Opinion: The 30 Gibbs is a lot of fiddle-dee-dee for about 1.5 inches less drop (26.5 vs. 28.0 inches) at 400 yards.]
--Bob
