Is Gain Block can be used a Power Amplifier
Whether something is usable as a power amplifier depends on how much output power it can provide, and how much you need in your application.

Look at the P1dB metric in the datasheet for each amplifier.  This tells you how much output power it can provide while just beginning to enter compression.  (You can squeeze another dB or two out of it by driving it harder, but it will have less gain and more distortion, so the P1dB point is a good reference).

The HMC637ALP5E has a considerably higher P1dB than the HMC311ST89, so comparatively the former would be considered more suitable as a power amplifier.  However, if you only need e.g. 15 dBm output power, it would be fine to use the HMC311ST89 as your final stage power amp.