You mean grade 8 is equivalent to 2 GCSE's in the amount of work that is required for it? It only helps you get into college/sixth form like GCSE's if you're doing music A/AS level and then you generally only need about grade 6 (and GCSE's at what grade?).

Grade 8 gets you about 55 UCAS points (which you use towards getting a place at uni) which is quite handy though! That's about the same points as an A grade AS-level or a D grade A-level.

But I don't think you can really compare music grades to GCSE's as they're totally different formats and types of qualification/achievement.