Are GPAs in the US based on averaging all of the marks from all of the classes in a course?
Generally, yes. There are some ostensibly harder courses you can take that are weighted slightly higher (the highest grade in an honors course may be a 5, while the highest in a normal course is a 4, so it contributes more to your overall GPA).
That said, I haven't ever really heard too many complaints about this system. In my experience the level of effort required from year to year is similar, it's the type of effort that changes. For example, in a first-year math course, you might struggle because you just don't know what you're doing. By a fourth-year math course, you may know what you're doing, you just have to do a lot more, and do it better.
What kind of non-engineering classes might there be in a engineering degree? In the UK courses are (or at least used to be - some have gone all "modular") rather narrow in focus - in the 4 year CS course I did in the 80s only one elective class was included that wasn't specifically related to maths, engineering or CS (and that was a rather enjoyable "History of Science" course).
The specific nature of non-engineering classes varies, but most science/engineering degrees in the US will require about 16 1-semester classes spread across {history, sociology, literature, etc}.
This is reduced a bit if you go to an engineering school. I only had to take 9 non-science classes (+ 4 gym classes).
That's true, and it is a specific complaint I've heard that I did overlook in my statement.
That said, I think it's common to specifically point out your discipline-specific GPA (especially) if it is substantially better than your general GPA, whenever given the opportunity (like on a resume).
Generally, yes. There are some ostensibly harder courses you can take that are weighted slightly higher (the highest grade in an honors course may be a 5, while the highest in a normal course is a 4, so it contributes more to your overall GPA).
That said, I haven't ever really heard too many complaints about this system. In my experience the level of effort required from year to year is similar, it's the type of effort that changes. For example, in a first-year math course, you might struggle because you just don't know what you're doing. By a fourth-year math course, you may know what you're doing, you just have to do a lot more, and do it better.