I have a friend who took the Grade C (Γ) exam. She has been here 20+ years, speaks fluently, practised her reading and writing a bit for the exam, and said it was OK but not a piece of cake. This level is supposedly about equivalent to the 'lower' in English as a foreign language, which most of the Greek kids pass at age 13 to 15, also said to be similar in standard to a GCSE O level (or whatever they are called these days
She's going to try for the Grade D next, which is more of the 'proficiency' level. She's a TEFL teacher herself and I think she's more sympathetic to her Greek students now as she realises how hard they work!
I don't know anyone else who has taken the exams, so can't comment on the first two levels. How about trying it Margarita??