generally it's a hard G before a broad vowel (a, o, u) and a J sound before a slender (e, i). Although like most rules in English there are plenty of exceptions.
Gas Giant is not alliterative. It is not sufficient for the words to begin with the same letter (nor is it necessary, incidentally); it is necessary and sufficient for the words to begin with the same phoneme.
Aaaaarrrrgggghhhh. Missed Great Gatsby because my eyes overlooked it when I was taking that quiz.... otherwise I would certainly have got it..... in fact it is one of the texts we are learning in English class.....
golden grahams
golden goose
golden fleece
golden corral