32 of this class were rebuilt from 4-cylinder compoundClass B between 1906 and 1917. The outside high-pressure cylinders were removed and the inside low-pressure cylinders were re-used, in their original position, to make a two-cylinder simple expansion engine. The boiler pressure was reduced from 200 psi to 160 psi to keep the tractive effort approximately the same. The rebuilt engines retained their old numbers. Additionally, 60 new Gs were built in 1910. The rebuilt engines were easily distinguished from the new builds by having "piano fronts".
The first of the class, LNWR No. 2653, was rebuilt to Class G1 in 1912. The remaining 91 engines were inherited by the LMS in 1923. LMS numbers were 9077-9144. The LMS rebuilt them all to Class G1 between May 1924 and 1937, thus rendering the class extinct.