I have seen many videos on youtube about the Greek Alphabet and everyone's question was What is the right pronunciation of the letters? First of all I'd like to tell you, that pronunciation doesn't really matters. I mean it's not so important if you say "beta" or "βήτα" Or if you say "Gama" or "γαμα" because when you put the letters in order to make a word, they will sound just fine. What they did, was that they replaced the ancient Greek letters with English characters. For example the letter "Y" ύψιλον was written "ὒ ψιλόν". So they replaced the first "υ" with "u" because it was looking pretty much the same and they haven't thought of the pronunciation which is a little bit different. In this video you see the CORRECT pronunciation of the letters.
Wherever you hear the letter G, replace it with W its closer to Greek pronunciation For example if you take the word «Γαμάω» which means "I f**k" you would probably pronounce it like "ga-ma-o". But if you replace "G" with "W" it sounds like "wa-ma-o" which is closer to the Greek pronunciation. By the way, the word "γαμάω" its not actually a bad word. It comes from the ancient Greek word «γαμέω» which means "I get married", but in modern Greek it is used as a swear.