A GP100 barrel - without question - will not fit a Redhawk, and vice versa. I wouldn't necessarily expect that the barrel thread diameter and pitch are the same, but they could be. The threads won't be the same length however, so I really don't expect it makes sense to even attempt - the forward member of the frames aren't the same size and the tenon penetration aren't the same. Since the GP100 has a smaller diameter cylinder and the frame overall smaller than the Redhawk, the mainpin/extractor rod cut on the barrel boss also won't line up. I could go on and on, but you get the picture - they are NOT interchangeable parts between models.
Ruger will typically be willing to rebarrel either or both, often the cost associated with this is more than selling your existing revolvers and buying new ones. They will ONLY create models that have been offered in the past, however. As Wave noted - $260 for rebarreling a simple round barrel revolver is way more cost than you'd be out to sell your GP100 used and buy another used one - heck, more than you'd spend to sell and buy new! Same for the Redhawk - $260 covers a lot of loss on used vs. used price risk. There's a bit of work that goes into rebarreling a GP100 or Redhawk compared to an SBH, but Ruger typically does take that work, as long as the end result matches a previously offered factory model (i.e. if your 4.2" Redhawk were a round butt, they wouldn't put a 6" barrel on it, as they haven't ever offered one like that).
Take-off barrels are around to be had for both, you just need to look around. If you don't see one on ebay currently, there are still several gun parts sellers on ebay that you can call that may have one. Numrich sometimes comes into barrels, but they go quickly when they do. These are typically $100-150, sometimes lucky to buy them for less. You do take a risk that it'll need to be set back a thread if either or both the tenon or shoulder are too short, but you can have a barrel installed typically for $50-100. That might be the cheapest option for you.
