Emil, welcome to the forum.
 
.40 or .45 cal? The original match rifles of the 19th century after which the Ped. Gibbs is styled were all around .45 cal. The .40 cal Gibbs is a new introduction and I think has stemmed from the success some black powder cartridge rifle (BPCR) shooters have been having with .40 cal rifles at long range. It also perhaps suits those who prefer less recoil. Despite grand announcements when it was launched the .40 Gibbs is as yet unproven in major competition although some good reports are starting to come in. .45 still remains the most popular choice. If you do consider the .40 cal be careful as Pedersoli manufacture two versions, one with a slow (1:24 I think) and one with a fast twist (1:16). You will ned the 1:16 for LR work.
 
International rules (see www.mlaic.org) and those for matches in Great Britain and continental Europe matches are fired:

• with metallic sights
  
• prone position and two point sling for ranges out to and including 600 yards
  
• beyond 600 yard prone with a two point sling and/or a wrist rest for support or back position
  

International matches are:

• 300, 500, 600 yards: 10 shots in 1 hour.
  
• 900, 1000 yards: 15 shots in 1.5 hours.
  

Team matches fired at the interationals are teams of four. Two matches:

• mid-range (300, 500, 600yds aggregate)
  
• long range (900, 1000yds aggregate)
  

There are some variations in these matches locally and in some US domestic matches cross-sticks are permitted.
 
The top shooters, well many are using custom built rifles and there are still a number of original match rifles being fired (particularly in the UK). The Pedersoli Gibbs has made the sport far more accessible and these are now major contenders - they are an accurate rifle and I have had great results from mine. People have however variously upgraded them with new sights, barrels and lock parts as suits need and pocket - but out of the box, providing you understand the proper managment of the rifle and have the shooting skills you will be a contender with one.
 
David