Duckhunter:
You will need to find a powder that keeps the case almost full up to the neck, the biggest danger with the Gibbs round is getting too much air space in the case. I used a lot of H1000, RL 22 & 25 and H450, the H450 gave me the most velocity but they all gave me good accuracy of 1/2 inch groups or less.
You also need to monitor the case head expansion, with the Gibbs if its put together right you won't have any hard bolt lift or the other signs of pressure, if you don't get ten or so loads with a case because the primer pocket gets loose you are to hot. I used to buy my brass from Grayback Brass to save wear and tear on the barrel because if you load these hot and the barrel is chrome moly you will have the throat shot out in 500 rounds or less. When I shot out the second barrel in my 25 you could drop a bullet in from the chamber end and it would fall 6 inches past the chamber before the rifling would stop it. The gun would still shoot into 1/3 inch but you had to clean the barrel every five or six shots. If you come up with a problem let me know, I had a Gibbs in the family for 10 years.