An apostrophe is needed if it belongs to someone tho doesn't it? EG:plural's eh?
Don't you mean G.Cs.S.E?Some might say that it should really be "G.C.S.E.'s", but the periods tend to get ignored unless the abbreviation conflicts with an actual word.
My manager and colleagues (who interviewed me) didn't ask about my GCSEs or A levels (had my interview on alevel results day last year). They were surprised when I said I did A level maths a few months ago even though it was clearly on my C.V.
I think it depends on what sort of job though.
Although I use "GCSEs" myself, I have no problem with people using "GCSE's", given that it's the plural of an anacronym (or in other words, the apostrophe is signifying that the word "Education" has been abbreviated).
Some might say that it should really be "G.C.S.E.'s", but the periods tend to get ignored unless the abbreviation conflicts with an actual word.