Canadians and Mexicans can apply for a NAFTA Work Visa (TN) that is simpler to get. You can apply for 1 to 3 years and you only need a letter from the employer validating that you have been accepted to work in a US company.
You also need to have some documents proving that you are qualified for the role. This includes diplomas, license, university transcripts, and a resume with relevant work experience.
Are specific parts of the transcript such as GPA or individual course grades of any importance, or is the completion of a degree + relevant courses in your specific field enough?
They generally ask for the full academic transcript (which has grades in it). Btw do not be worried if you don't have stellar grades/GPA - USCIS doesn't care about that. Your immigration lawyers just need to make a case that your education plus work exp (if any) is relevant to the job the company is hiring for.