Planning on taking a gap year but unsure

I want to take a gap year after college ends before uni to work full time in order to save up money to pay for my degree (bachelors), only problem I’m thinking of is how do I not forget key knowledge I need to start my degree? Do universities offer any help regarding that? What should I do?

What's the subject you intend to do your degree in?
If it's a STEM subject, some unis would be mindful to my knowledge. They would expect you to have done things in the gap year to sharpen your knowledge.
For some degrees that don't stress on subject specific requirements and expect you to know nothing prior to starting generally don't seem to mind as much.
As far as I know, universities don't offer any help per se.

I intend to study engineering, most likely civil engineering to be specific. I assume I’d have to remember things like calculus then

Either go through A Level Maths again or pick up university level engineering maths books to work through (first year books of course).
The book that comes highly recommended in this area is Mathematical Methods for Physics and Engineering by K. F Riley, Michael Paul Hobson, and Stephen John Bence
I can't specifically tell you which chapters are relevant for civil engineering, so you would need to do the research for this yourself.
Another good series of books is the Schaum Outlines series. You will need to find out which maths books are relevant for your chosen discipline though.