They can't be directly and formally equivalent - grade 4 & 5 pieces would be considered "more difficult" pieces by exam boards marking your performance though (and a grade 5 piece played to distinction level would easily get you an A*). This is for performance only, however.
A much better guide would be grade 5 practical and theory together, which would (in most places) enable you to start studying A level music if you hadn't done GCSE. However, the standard you need to ultimately reach at A-level for high grades is obviously higher than G5, and having Grade 5 theory & practical doesn't mean you have all the knowledge you gain in GCSE - e.g. I found I had all the knowledge I needed for performance & the parts of the listening paper where Western classical theory was needed, but that doesn't mean I knew anything about, say, Indian classical or Britpop.
So basically, Grade 5 dist. could easily be seen as equal to or better than A* GCSE performance and Grade 5 theory & practical could be seen as vaguely equivalent to GCSE but there's no formal equivalents. If you have Grade 5 practical and decent musical knowledge though, you're probably more than capable of an A* anyway though.
Hope that helps, it made my head hurt!