Converting Unix epoch time to GMT time using QGIS Field Calculator

In the QGIS python console, you should be able to run something like:

import datetime
datetime.datetime.fromtimestamp({your epoch time}).strftime('%c')

Note that strftime can be formatted in many ways, but the time zone will be UTC. For instance, you could instead use the following: strftime('%Y-%m-%d %H:%M:%S') (See strftime for the possible formatting values)