As you say, since Contax G → Sony E-mount focusing adapters do exist, so it's entirely possible to build one for Leica L-mount (SL/T). According to the Wikipedia table of flange distances, there's about 10mm to work with between the mount depths (Contax G=29mm vs. Leica L=19mm).

But, volume-wise the adapter makers probably don't find it worthwhile to make them, since mirrorless Leica owners are probably thin on the ground. And, unfortunately, since the Leica L mount is roughly the same depth (19mm vs. 18mm) as Sony E, you won't be able to find a Sony E → Leica L adapter. And the micro four-thirds and Fuji X ones might vignette (if they existed) since they're not for full frame.

You can't stack on an SLR lens adapter because you won't find Contax G → SLR adapters (just as you won't find them for Leica M), due to flange distances on SLR mounts being much thicker than Contax G.

I think you may have to find a custom service to convert the mount on the lens. Or just wait and see if someone does come out with an adapter. The main problem is that it's going to be very expensive if one does. The cost of the Sony adapters has been averaged out as they've already reverse-engineered the electronics of the focusing and then just been able to use that for additional adapters. Having to start over from scratch with an entirely different mount for low volume sales simply may not be worth the effort for Kipon or Metabones. And, making a low-cost ring isn't going to be worthwhile, if you want some way to focus the lens, given Contax G's lack of focus rings on the lenses.