Hello!
I've just recently graduated from University in the UK and I had enrolled as a student studying in international school, so I have studied both the GCSE (year 9 to 11) and the I.B diploma (year 12 and 13) respectively. However what I found was that I still needed to take the IELTS exam eventually despite having english as my native language and taught language throughout my secondary education.
IELTS is out of 9.0 and many universities expect at least a 7.0 for undergraduate degree courses, and possibly 6.0 for a foundation year course. My first attempt was a 7.5 and this was with no preparation and as mentioned above this caught me off guard. I finished university and this time did a few past papers and resat the IELTS and received a 9.0, being a native speaker definitely helps but if you want/need the extra marks exam technique is where the difference will be made. I know there are many who will be in disagreement here but for me personally IELTS was merely a means to an end (to get into university).
Essentially, echoing what has been said already, GCSE english took me the 3 years in school to learn and study, IELTS I had to do as a requirement (despite having GCSE and I.B diploma) but the difficulty was definitely not the same and I could prepare for it in a few days. My friends at university studied in local school and english was a second language for them but doing the past papers for IELTS also helped them get the grade they needed.
GCSE- You really learn english, it can even be (usually is) challenging for even native speakers as literature isnt for everyone but you gotta do it with language (so we had Eng Lit and Eng Lang)
IELTS- You cant really choose because if you're aiming to study in UK they will most likely ask for it anyway, but its only valid for 2 years so dont do it till you start applying (I did mine after I got an offer)
Sorry for the long post hope it helps!