Is ganska similar to the english word "moderately"? As in, he is tall, but he's not very tall, so he must be moderately tall. (A more colloquial way to was "moderately might be "kind of"). But do these words fit within the meaning of ganska?
I think it's not all too fitting to translate it too "moderately", but it's not much of a stretch either. I feel that "moderately" would rather translate to "måttligt" or "någorlunda".
is "ganska" somehow related to the german "ganz schön" e. g. "Er ist ganz schön lang."?
It's historically related to ganz, but it only means ziemlich. (these days, the meaning has changed over time).
'Very' and 'pretty' are not necessarily synonyms. They definitely have different implications. Very tall almost implies taller than most but pretty tall can just mean taller than the average person.
In my opinion, not quite -- although it always comes down to semantics :). I believe that "pretty" is a weaker modifier (think "rather" or "quite" -- the person is not very tall, but only pretty tall), whereas "really" is a synonym of "very." Although the difference is super small, I wouldn't translate ganska as "really".
"Pretty" meaning "fairly" is a colloquialism, doubtfully suitable for a language course.