If you don't have good gcse will you find a good job?Watch this thread
Its important to have GCSE Maths and English because employers expect them, then the next target is 5 GCSE as that can be a requirement for some courses. Some courses and unis demand more than that, but in general 5 would be ok.
You are also likely to need a B if you intend to study A level, but not for an Access course or BTEC.
Its possible to resit GCSE, so if you dont have the above then a bit of hard work and you can get them. If all you have is GCSE then you can work up, but if you have enough to do Level 3 courses then you can still get to uni and the higher and better your qualifications then the less GCSE will matter. They will become more important to get a job.