The database Dissertations & Theses is an online index of master's theses and doctoral dissertations from universities worldwide.
The library does not own most of the dissertations in D&T. Once you've identified the dissertation you need, we may be able to get it for you by Interlibrary Loan (but see Open Access Dissertations below).
The ScholarWorks @GSU is GSU's online repository of dissertations, theses and faculty publications available for anyone to view. It mostly contains recent dissertations and is not a complete collection of all existing GSU dissertations.
You'll also find some books about dissertation writing: you can eliminate these from your search by choosing "Manuscript" format from the left column in a GIL-Find search.
The library mostly owns dissertations by GSU authors. For non-GSU authors you'll usually need to use Interlibrary Loan.
Other universities may have their own equivalent of the GSU Digital Archive, in which they share dissertations and other university publications online.
These repositories are maintained by individual university libraries or archives and the best way to locate them is usually to simply google "dissertations" plus the name of the university.
If you identify a dissertation in the Dissertations & Theses database, check an online search engine such as Google or Google Scholar to see if the full text is freely available online -- you may not have to wait for an Interlibrary Loan request.