The sound of gamma before back vowels is [ɣ], which does not exist in English -- it's the voiced version of χ before back vowels which is the "ch" in "loch" or "Bach" for some speakers.
This back gamma sounds similar to the "r" in most dialects of French or German (Greek: velar fricative; French+German: uvular fricative).
It isn't the "g" of English.
This was the hardest sound in Greek for me to get used to. (χ was fine, maybe because I learned German when I was younger).
Isn't French / German 'r' usually uvular rather than velar?