Columbia College (CC) is the Ivy League undergraduate college of Columbia University, currently ranked number 4 (behind Harvard, Princeton, Yale) by US News & World Reports. Admissions percentage for the class of 2016 was 7.4% When high school students are applying to Columbia to be admitted as freshmen, they are applying to Columbia College (CC) of Columbia University.
GS is Columbia University's School of General Studies. It is intended for those who are non-traditional applicants: adults resuming their educations; older applicants who may not previously have attended college, or were perhaps in the Armed Services...GS students are not the "typical" fresh-out-of-high school applicants or, if accepted, Columbia undergraduates. They are admitted by the school of General Studies through an admissions process completely separate from either Columbia College or the undergraduate engineering school. However, GS students take most of their classes with CC students and are held to the same high academic standards, and curricular rigor, once admitted. They bring their own extremely valuable perspective to the classroom. GS is NOT the administrative unit to which a prospective freshman, applying in high school for college, would apply.
Columbia University has an extremely user-friendly, detailed website which describes all aspects of its unique curriculum, and it will answer any questions you might have about admissions to either CC or GS.