Is anyone aware of such a thing? The only chromoly bike I'm aware of that takes the Gates is the REI Gotham which, as far as I know, isn't available as a frame.

Oops, make that centertrack.

Perhaps, several terms being mishmashed together ?

Gates center track is the 2nd version of their belt

Drive center line ?

that is determined by where the Hub places the toothed ring,

you adjust the crank part of the chainline to match It. ..

chain line is same thing were it another IGH , with a chain.

Compatible frames .. have to Open the rear triangle , to let the belt pass through, some how.

many other frame builders can offer a useful frame, but may not be as cheap
as the one REI brings in.

https://www.rei.com/product/825377/no...tham-bike-2013

interesting combination .. with continuously variable Gear Hub

The Civia Bryant should be available as a frame set, and has gotten some good reviews. Civia is distributed through QBP, so most bike shops should be able to order it for you (or online at a place such as universal cycles)

Spot Brand bicycles is belt-drive centric, but I'm not sure if you can get a frame only.

Many pre-existing chromoly frames can be re-worked by a frame builder to accept belt drive by cutting the drive side chain stay and inserting a Paragon stay splitter.

Other companies are producing bikes with the Gates system, but they are typically only available as pre-built bikes.

Van Dessel - Whiskey Tango Foxtrot
Speedhound - The Only One
Many Tout Terrain bikes
Raleigh Hodala