Rocky Gibbs developed the Gibbs line of cartridges in the early 50s. Based on the 30-06 case, the Gibbs was in essence another presentation of an improved cartridge. Objective testing of the 30 Gibbs has produced velocities in the 2,900 fps range with 180gr bullets, 24" barrel. This is about 300fps lower than what Rocky claimed, however, chronographs and a better understanding of pressures have come forth in the intervening years.

Bottom line is that the 30 Gibbs, like the 06 Improved, will develope 50fps to 100fps over the parent cartridge in barrels of equal length under equal pressures. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.