The first syllable is in 'jenta', while none are in 'gjenta'. 'gjenta' is also a compound word (gjen+ta), so the 'n' can last longer than in 'jenta'. The easiest way to differentiate would be to put the word into context. Would "Can you the girl the question" make sense in English?
As I said, there is a slight difference in pronunciation, but when you're hearing fast speaking, you might not be able to hear this. So it would probably be easier to just understand by context, but if you were to listen to both words some times you might be able to recognize this difference.