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