You could try googling with terms like 'phonics' and 'hard and soft G.'
Children are sometimes taught that G is usually hard before A, O, and U
and gallon, sugar, goal, dragon, gum, guard, plague
and often soft before E, I, and Y
general, age, geology, giant, imagine, gyroscope
But you can see that there are many exceptions, from basic words like 'girl' and 'get' to more complex words like 'gynecology.' And there are other ways to spell the same sound, like J (jelly, jump) and DG (edge, fudge).
There are also complications if you consider words that come from foreign languages, like 'garage' (as in French), or 'gyro (sandwich)' (like an H).
And of course NG isn't always the same in English -- it's sometimes one sound, like German NG (singer, ringer), but sometimes two sounds, like NG + hard G (finger, linger).
Sorry. Just remember how hard the umlauts and German R's are for us. (-: