This has to be implemented in the source (autoDJ) software, not in Centova Cast.
sc_trans2 already has support for applying replay gain values, but we removed its configuration options from the Centova Cast UI prior to shipping the first sc_trans2-enabled build for some reason -- I honestly can't remember why, but we had some kind of problem with it. We'll have to revisit this and see about adding the options back to the UI.