It depends on the programme really. May I ask which one? Incidentally Imperial and UCL do a joint degree in MSc Transport - which may or may not be of interest to you. On average, there are 7 applicants for 1 place at postgraduate at Imperial - this of changes depending on course (with some courses rising to 20 applicants to 1 place). But don’t let this deter you.
Usually the minimum requirements for entrance into Imperial is a second class upper division (2.1 degree) undergraduate degree. It’s difficult to convert this to GPA as the systems do not exactly line up. However Oxford rate a 3.7 GPA as a high 2.1. Imperial also weight your grade by the selectivity of your school - Berkley is a selective school so the GPA requirement should be lower. You should aim for a minimum of 3.3-3.5 GPA. However, it must be noted that this an absolute minimum. In my experience, most people applying will have first class degrees or very strong 2.1 degrees (ie 3.7-4.0). Again it’s not an exact science. Some may even have masters and PhDs already - this was the case when I was at the LSE. But don’t fret too much, entrance into UK universities is mostly determined by grades. So if you have great grades you’re already over halfway there.
Secondly, you need a strong statement of purpose. UK schools are not interested in your swimming or chess hobbies. In it include extra-curricular, courses, research, work experience and insights relevant to your chosen degree. And only those relevant to the degree. You should explain how you plan to use the course in your career or as a means for further study - link it back to your research interests. Talk about why you like the course, and perhaps pick out modules. It should be passionate, but professional, of course. I’m sorry if this comes across patronising - I only spell it out just in case!
The other matters (like references) are stated on the website. I will just say, if you are going to apply think about doing it sooner rather than later. Masters programmes run on a rolling admissions system here. So the earlier you apply the more likely you are to succeed. Also consider the potential impact of COVID - we are in our third national lockdown, however we are ‘successfully’, by the standards of our incompetent Tory government, deploying a vaccine rollout. But seek the official statement from Imperial on the state of their in person teaching next year.
Best of luck!