The reality is that you need reasonably good maths to handle both the job and the qualifying examinations.
GCSE in itself is only an indication of that.
If you did very badly in GCSE maths, then that is presumably because you haven't learned the maths very well. So, in my opinion yes, you need to go back and improve your grades - not so much because the grades matter (although they may), but because you need better maths.
I'd go further and suggest doing A levels and trying to do at least the lower level of A level maths, because realistically the maths in the ATPLs is at the top end of GCSE skill level. That is easier said than done however if both you struggle with the subject, and the school want to up their position in the league tables.