Hey there, I've been in your place and clearly understand the nerve and stress students experience when it comes to self-assessing whether there GCSE predictions are good enough for their chosen universities. Firstly, determining whether universities' decisions are majorly influenced by your GCSE grades/predictions is a very broad topic as each university has its own selection criteria (that it would expect its applicants to meet), and furthermore the selection criteria differ in type and difficulty with respect to your chosen course. For the majority of higher education courses the expectation is that you achieve the standard English literacy and numeracy requirement (that is a 5 in English language, yet some universities will consider English literature as an alternative, and a 5 in mathematics). This is the minimum expected standard. Depending on your chosen course you'd be expected to have excelled in certain GCSE subjects, which again this depends on the university in question. For example, I have applied to study medicine. Three of the universities i have applied to expected me to achieve a 5 in my English language, yet one asked for a specified number of 7s, preferably across my sciences (As medicine requires the utilisation of scientific knowledge).
In conclusion, even though the answer to your question is not complicated, it is dependant on a lot of factors and there's no one specific answer to whether your GCSEs are majorly important when it comes to applying to university. To ensure that such a question is accurately answered I'd advice you to contact Manchester University and inquire about the GCSE requirements for your chosen course. Alternatively, you can visit the academic criteria on your university's web page yourself. Feel free to reply should you have any other queries!!