Never really have been any statistics compiled that demonstrate that it definitively does anything better than standard rifling, same as variable depth and Polygonal rifling (elliptical bores, round bottom/radiused rifling, modern 5-7R rifling) (least as far as I'm aware of). You can still use heavy charges in a BP rifle (or a smokeless rifle using Red Dot (which has a burn rate similar to 777)) with standard rifling, you just have to ensure that your projectile has enough bearing surface to hold onto the grooves. Just look at the BPE rifles, like the 577/500 BPE No. 2 (3-1/8"), which was often loaded with 165gr of BP and a 400gr bullet or .50-140-700 WCF (I know they're chamberings from after '65, but it is relevant). The guys selling gain-twist barrels claim it works miracles, but can't ever seem to back it up with anything; the guys that use gain-twists believe they do wonders, but often don't outshoot anyone else.
Gain twist is still offered by some barrel makers for modern guns, but much like historical shooters, people won competitions with standard rifling, variable depth, and gain twist. Just like in the 1800's, most modern shooters don't bother with it, as it is more finicky (and expensive to produce) than standard rifling, for no definite advantage. 1860 Colt's used gain-twist originally (it was the "wonder-tech" in rifling at the time, a product of the dawn of modern engineering/mathematics), yet new repro's don't, and they shoot just as well; and some target rifles used gain-twist, but some new repro's using standard rifling are shooting just as well in 800 and 1,000 yard comps. Much of the draw to gain-twist was based in the same logic James Forsyth was running with in his book: that a faster twist somehow drastically reduces the velocity and effectiveness of a projectile, due to excessive friction and the energy being changed from forward movement to rotation. That was mostly disproven by mathematics and trajectory studies in the 1840's and onward (ironically at the same time JF was making his claims), while you do lose some velocity from faster twists, we're talking 50fps or less, not the hundreds JF claimed.
Not to drag the thread into smokeless (which we're not really supposed to talk about), but modern smokeless rifle powder has been formulated to burn slower to allow us to produce more gas at the same pressures, to accelerate a bullet to much higher velocities without requiring a significantly stronger/heavier gun (that developed out of the experiments that began in Europe, Canada, and the US all the way back in the 1840's, trying to make a "better" gunpowder). That slower burning and lower pressure impulse also is what causes issues with bore-sized projectiles in SML's (not the topic of this forum). BP is honestly THE best powder for ML's, as the sudden high pressure impulse of that explosive upsets the bullet into the grooves very well, but it doesn't require gain twist to work well, even with high charges, so long as your projectile fit is proper.
So long as the RPM the bullet is spinning at once it leaves the barrel, it doesn't seem to matter how it gets there.