What to include in a cover letter
There are some 'career fact sheets' about cover letters in Career Hub as well as books you can borrow from the Reid Library.
In terms of attaching a copy of your transcript to your application you probably only need to do this if the company specifically requests it. Otherwise, simply ensure within your CV that you note the broad fields of study associated with your degree major/s as well as your level of achievement (WAM/GPA).
If you do not feel confident that your GPA is one of your strengths then focus on your skills, however, if a GPA is specifically requested you will need to include it.