Please Note in the above picture on the Right side: Left to Right for size comparison: 6mm GT, 6mm Creedmoor, 308 Winchester. 

One Cartridge, not a box: 6mm GT or 6GT produced by Hornady Manufacturing Company in Nebraska, USA (for GAP) factory loaded using new Hornady Brass cases with a *“GAP 6mm GT” Head stamp and loaded with a 109 grain Hornady ELD Match bullet.  
* (Headstamp: GAP for G.A. Precision and GT for George and Tom, see below)

History of the 6mm GT or 6GT Cartridge:
The 6mm GT or 6GT was design and tested  as a precision  rifle competition, wild cat cartridge by George Gardner (Owner of GAP = G.A. Precision) and Tom Jacobs (Owner of Vapor Trail Bullets) in 2019 for custom rifles by GAP.  
(I would think the GT in 6mm GT is for George and Tom)
With 3,000 Feet per Second muzzle velocity using a 105 grain bullet.
The cartridge was approved by SAAMI on 01/16/2022 and became available by Hornady to the public in late 2022 .