You can change majors and you get admitted to GT as a whole, but it may be difficult to transfer into CS if you wait more than one semester, I have heard. Also if you start out taking another major’s requirements, it will take an extra semester or two to finish, making changing majors costly at GT, if you are from out of state.
Registration can be more difficult in crowded majors at GT, but gets easier as your seniority goes up.
I don’t think there are quotas per major, but I don’t know for sure.
The CS major, because of its high rank, has a much higher number of out of state applicants and international applicants, that designate a CS major from the beginning,
See Freshman profile for class of 2014 Computer Science, including Computer Media majors, in 2014: there were 123 Georgia students enrolled, 136 OOS students enrolled, and 44 International students. I cannot find later data, maybe someone else knows how to find it. It may vary year to year.
See Page 3 Freshman Profile for CS majors:
https://www.cc.gatech.edu/sites/default/files/documents/coc-undergrad-book-june-2015-web.pdf
Here are the sizes of each department, over the years, and both mechanical engineering, the largest major, and CS major are growing, so the school is now a total of 15,000 undergrads.
http://factbook.gatech.edu/admissions-and-enrollment/undergraduate-enrollment-by-college-table-4-16/