Whats the difference between amplification and gain in a BJT?
The current through a BJT is controlled by the base-emitter voltage VBE, it depends exponentially on that voltage. From this point of view the BJT is a voltage-controlled device. 

The way a BJT works makes it necessary to also have a small current through the base. This base current is only a small fraction of the collector current. The ratio between these currents can be used to define a current gain and it is possible to regard the BJT as a current-controlled device.

So we have two ways to deal with a BJT and depending on the situation one is better than the other.

For a CB circuit we see that input and output current are almost equal and it could be used as a current buffer. Apart from that current gain doesn't help much to gain further insight. 

Using the voltage-controlled operation approach, we see that the emitter gives direct access to VBE of the transistor in almost the same we as for the CE configuration, only the sign is different. So a change of the emitter voltage causes a change of VBE. This also changes the current through the transistor  which is then converted into a voltage by Rc. So it is possible to achieve voltage amplification.

However, usually voltage amplifiers with a larger input impedance are wanted and therefore the CE stage is preferred over the CB configuration. This leaves the use as a current buffer like in a cascode circuit.