From my experience, the sixth forms and colleges around me wanted at least grade 6 in the subject I wanted to take, and also a grade 6 in either English lang or maths (depending on if it was a science or humanity subject eg. History I needed either history 6 and English lang 6 or English lang 6)
Also it can be subject specific what grades matter and not at GCSE — I took history, English lit and economics, but I got a 5 in my maths, 6 in English lit and 8 in history at GCSE. I now find economics my easiest subject, and English lit my hardest.
Your grades won’t reflect how well you know a subject, but it may be applied by colleges and sixth forms for how well you understand the subject related to the one you want to study for a levels. They simply just need to put a barrier to incentivise students to do better and to help them understand subjects relevant to those “needed” to study your preferred subject at a level. But if you’re lucky, you can find plenty of places that don’t care so much about grades and get in easy. Just work hard at a levels though — it’s tricky!!