@Leonab I can certainly see that it won't go well if someone is using a tutor and essentially expecting the tutor to do all the work for them, not taking the tutor's advice on what to do between lessons, and imagining they will emerge with a nice set of GCSE results.
However, not everyone learns that way. Some don't choose to do GCSEs at all, or not when they are young, instead going straight into work. Others approach exam preparation in a manner which I'm sure you would consider slapdash, but which is effective for them. For example, here's how my eldest child tackled maths.
They did no formal maths as a child, instead learning through whatever interested them. Money is an introduction to decimals. If you like science or wonder about converting fractions to decimals, you'll encounter more. Probability is important to kids who like to play D&D or poker. Converting American recipes gives experience of different units of measurement. We never have the right size and shape baking tins, so you need to know how to calculate the area of a round tin and see whether it's close enough to the area of the rectangular tin specified in the recipe. And so on. They had achieved basic numeracy by their mid-teens, though their knowledge didn't map perfectly onto the school maths curriculum.
My child wasn't originally planning to do maths GCSE. When they decided to sit the exam after all, they downloaded the syllabus, identified the topics which were new to them, and figured out which of those they actually wanted to learn which would bring in enough marks to get the result they wanted. They didn't bother to learn topics which scored minimal points and which didn't seem relevant to their life, reasoning they could learn that material in later years if needed. They obtained a textbook, learned the target topics, and sat some mock exams. Once they were comfortably on track for the result they wanted, they stopped working.
The overall picture here is not of a disciplined, hardworking student who would have pleased schoolteachers. That wasn't what my teen aspired to be. They wanted to know only the maths which was fun and interesting, plus the maths they perceived to be immediately useful to them. Having decided to sit the GCSE, they sought to do the bare minimum for the result they reckoned they would need in life. I didn't notice them experiencing any particular difficulty from the "knowledge gaps" in maths which you mentioned. They saw them; they fixed them, starting with 7x8 which had somehow never sunk in previously. It wasn't a big deal.
You don't have to be a model student if you aren't spending most of your time in a school-type environment. It's possible for learning to be fun and playful most of the time. In the home ed community, GCSEs are seen as a (usually) necessary evil, and some kids adopt an expedient approach to getting through them. Even for an arts-focused child like mine, there are plenty of opportunities to learn more maths throughout their life, provided you haven't absorbed the idea that maths is so unpleasant that people would only do it if forced, which is the message instilled by school.
A kid who isn't motivated by a school-type approach is, I think, a particularly good candidate for home education, which can give them the scope to engage with their learning on their own terms.
If you haven't read Lockhart's Lament, I recommend it to you: https://maa.org/sites/default/files/pdf/devlin/LockhartsLament.pdf