I'll answer your question directly and then throw in my $.02. Most schools look at your entire GPA AND your major GPA. They use this as part of their holistic approach at gauging you as a potential contributor to their research base. Some schools do not care about your major GPA, they want to...
Mechanical engineering is inundated with Mathematics and Physics. You will be using Mathematics every step of the way. At some point, a teacher may start using the dreaded word "Qualitative" and you will be disgusted by that, if you're anything like me and my fellow students.
I'm a grad student in Mechanical engineering. I was, like you, undecided in the beginning of my undergrad studies which engineering field to tackle; I was torn between Mechanical, Electrical, and Chemical. I chose Mechanical because, like MEM33 said, Mechanical is very broad. This is a good...