Gain block vs power amplifier
There is a lot of overlap in what manufacturers describe as gain blocks and power amplifiers in the +10dBm to +20dBm output region. It would be an unusual power amplifier with less than +10dBm output, and gain blocks don't often have >+20dBm output power.

As well as being lower power, gain blocks tend to emphasise good port matching and good isolation, as well as easy biassing.

Power amplifiers tend to emphasise better efficiency, even if at the cost of output match and simple biassing.