I see now where the confusion comes from: the abbreviation RTL is overloaded to (1) register-transfer level (as e.g. Verilog) and (2) register-transfer language

I see now where the confusion comes from: the abbreviation RTL is overloaded to (1) register-transfer level (as e.g. Verilog) and (2) register-transfer language (e.g. GCCs intermediate representation you cited). I think the former use is much more widespread. I agree GCC's register-transfer language is close to Sail.