Sixthform gcse retake

I’m a sixthform student who just resat my maths gcse test and I want to go through clearing but since the days for alevels and gcse results are different I don’t know what to do. If I wait till my gcse result, most of the places on clearing will be gone. I have a firm choice but I don’t want to go there anyone, I’m not quite sure what to do.

The vast majority of sixth form students who also resat a GCSE or two alongside their A levels / BTECs will have done so at the end of Year 12. So it's fairly unusual for someone to still be waiting for GCSE results on A level results day, I'd have thought.
You could always apply via Clearing tomorrow for courses where there are no GCSE Maths requirements, or where your original grade meets the requirements.
Failing that, I think you'll just need to wait a week and hope that some courses you're interested in are still available.
Just out of interest, is it your firm course that you're no longer interested in, or your firm uni?

Hey I did go through clearing and got into Exeter and my course doesn’t require maths but I’m a bit worried that they’ll see I don’t have maths and take the offer back.

If your course doesn't require Maths, and you've formally applied via UCAS Clearing (i.e. not just received an informal offer over the phone or via email), then they know you don't have Maths. So they'd have no grounds to withdraw to rescind the offer. Stop worrying.

I received my offer through email and not through ucas.

OK. So this was the result of phone call with the uni, or perhaps completing a form on the uni's web site - right? In that case, it's called an "informal" offer. This is because it's based solely on the information you've provided and so is not binding on the university (as there may have been misunderstanding on the phone or whatever).
The next step is to log into UCAS Hub and enter this uni / course as your Clearing choice. Don't delay in doing this, because a informal offer expires fairly quickly (perhaps 24 hours, perhaps sooner). So read the email carefully to find out when it expires, and make sure you've entered the course as your Clearing choice into UCAS Hub by then.
At that point, the uni will get to see your full UCAS application (which they've likely not seen until now)? If everything you've told them already matches what's in your UCAS application, then they'll confirm your place in Clearing and you're definitely in.