I downloaded the Common App’s document for the Transfer Mid-Term Report, and I was wondering if any of you knew if it has to be filled out by the actual professor, or if a GSI/TA that does the grading will suffice? Any insight would be helpful 
Only professor should fill your Midterm Report. No TA or GSI. That’s what I know.
In case any future transfer applicants are looking for this information, I contacted the individual colleges and it really seems to vary college-to-college. Some say it’s fine to have your GSI fill it out, others say that GSIs can fill it out if the professor signs it, and others say only professors can fill it out. I ended up having my GSIs fill it out with a professor’s approval and signature to be safe. So, just make sure you check what the requirement is with each school