I agree that in general the GT's OST's aren't as good as the Z's one, but yeah, the opening and ending were rather great melodies, at least the ending I am thinking about (I know the opening is kept constantly which isn't true for the ending). The Genki-Dama soundtrack (which is basically the instrumental/theme version of the opening played slowly) is one of the themes that I think is the best of all times, plus its nostalgia feeling that gives me whenever I touch upon it... dunno, it just does, and I feel relaxed and somewhat melancholic whenever I hear it.