Current gain or voltage gain?
The fundamental action of a bipolar junction transistor (BJT) is current gain. A small current passed through the base-emitter junction allows a greater current (maybe 100x) to pass between the collector and emitter.

But a BJT is essentially never used by itself, rather it is one component in a circuit, often an amplifier circuit.

By passing the greater collector-emitter current through a resistor, a voltage arises across the resistor. This voltage can be substantially greater in size than the input signal applied to the base of the transistor. So the circuit in which the BJT takes part can exhibit voltage gain.

There are other interpretations of BJTs as a voltage-controlled device (rather than current controlled), but that's for a later chapter.

As a first concept, the BJT is a current amplifier, which deployed in the appropriate circuit, can imbue that circuit with voltage gain, making it a voltage amplifier.