Hello,
I do not know which method is used to rifle the Gibbs brrel but it is probably buttom rifling. I have seen notes on other sites that indicate the bullet mould made by Pedersoli actually does not fit the Gibbs rifle and bullets from it have to be sized.
With regard to custom bullet mould makers, there are two in the United States that most folks discuss. They are Paul Jones and Fred Leeth. Both make excellent moulds that turn out match quality bullets. If you go with a custom mould you definitely need to slug the barrel. The Gibbs bore is slightly smaller in diameter than a standard .451 bore.
I believe the last 1000 yard match winner at Oak Ridge was shooting a Creedmoor style grease groove bullet from a Fred Leeth mould. I think the weight was in the 540 grain range.
Another consideration for you is that many of the long range Gibbs shooters are shooting paper patch bullets in the 540 to 560 grain range. These bullets do perform very well at long range but there is more cost for a swage and dies and more effort involved in making them. They do make a neat looking bullet.
Hope this gives you something to think about.
Colin