The voltage gain comes from the split resistor on the right. It basically reduces the emitter degeneration effect on the transistors. The only problem is that without a transformer, you can't galvanically separate the input source from the output. Basically the input must be referenced to the mid-point of the split resistor, rather than ground. This makes the circuit unsuitable for combining with other stages on the input side. The output is ground referenced, so it's not an issue to be followed by other stages, just not preceded.
