Non-native speaker here, what does "hard G" even mean? G as in "go", or G as in "gin"?

Hard G is "go". The name of the format if a play on words, in the US there is a brand of Peanut butter called "Jiff". The file format is supposed to be pronounced the same way (soft G). Some people, like the OP, claim that the G is supposed to be pronounced like the G in graphics (hard G). IDK why anyone cares.

"G" as in "gadverdamme".

(Native speaker here - but of Dutch, the language known for its proper hard Gs. Unless you're from the South.)

"hard G" is the sound denoted "g" in IPA. The other "g" is "dʒ".

That is the convention in english as least. Other languages may differ.

Fortunately, since we are explicitly talking about English here, that convention is indeed the relevant one to use!