Yes, in med applications GCSEs are super important. As the vast majority of applicants don’t have A levels at the point of application then the majority of schools base their sifting to screen for interview on GCSEs and UCAT (and possibly contextual factors) in some combination.
If they state a GCSE profile is a minimum requirement, then you must have those GCSEs or you won’t move to the next stage - (unless you have strong extenuating circumstances which they accept).
Some unis also state a minimum UCAT score, sometimes a threshold, but more often it’s a minimum SJT band.
If you don’t have the basic entry requirements (incl A level subjects) then you won’t get past the door.
Sadly it doesn’t guarantee you an interview if you meet the entry requirements as there are so many applicants. So they then have to rank or assess everybody who applies and meets the entry requirements to decide who to invite to interview. So they may do some type of formula or scoring of GCSEs/UCAT. Only those that meet that next level will be invited to interview. So it is extremely important to apply strategically.
You can cut some out if your A level subjects aren’t right or you don’t meet the GCSE requirements and then assess the rest to see your best chance of interview when you have your UCAT score. Read the application policies on their websites.
And put your info into the which medical school thread when you have your UCAT to sense check your options.
Good luck.