Lew, the term "gain" is commonly used to refer to "voltage gain," the ratio of output voltage to input voltage. It is often (although not necessarily) expressed in db, corresponding to 20log(Vout/Vin). So in that sense transformers can supply gain, although as you realize they cannot provide "power gain" (i.e., they cannot increase the product (multiplication) of voltage and current).
Best regards,
-- Al
Best regards,
-- Al