Hi,
I just took my IGCSE Music exam (the "I" just stands for "International" I guess... I live in Hong Kong), but the syllabus should be the same as the one for GCSE Music:
30%- Performance (This section is recorded and sent to the examiners- it requires you to play a piece of your own choice for one solo, and then another solo or a group performance with 2 or more people for your second recording. The total performance time for both pieces must be between 4 and 10 minutes- not hard!)
30%- Composition (Three of your own compositions [could be of any length] have to be submitted by the end of the 2-year GCSE Music course)
40%- Listening (There are two parts- unprepared listening and prepared listening. Unprepared listening is 1 hour long and asks about theoretical knowledge [i.e. which composers from what period, musical texture, etc], and is followed by Prepared listening, which is 40 minutes long and asks about 1 or 2 pieces of chosen music which have been already prepared beforehand [for this you'll need to know about the instruments, period, composer, etc- theoretical stuff again].)
An ABRSM standard of grade 5 or above for at least one instrument (or just your singing voice!) is recommended, although it's not compulsory.
Although it looks demanding, GCSE Music is great because all the hard work is spread out over a 2-year course- if you can play only one instrument yet still think that you're pretty good at it, plus you can read music notes, you should definitely give it a try.
Hope this helps.