Can you increase your undergrad gpa after graduating? I've googled this question a few times and I've seen multiple different answers. I read that you can raise your undergrad gpa using a post bacc and I also read that post baccs provide a separate gpa. So, I'd really appreciate if I could get a definitive answer, preferably with a source. Thank you.
Yes, it is possible.

Faculty at my university have the right to change their course grades up to five years after the course ends.  I have a standing offer that if any student publishes a research paper as a result of work in my class, I will retroactively change their grade in that class to an A+.  I have actually done this a few times, in one case two full years after the course ended.

(All the students involved in those changes already had A's, and my university counts both A and A+ as 4.0 in its GPA calculations, so the change did not actually change their GPA.  But in principle, it could have.)

But this is the only way I can imagine a student at my university changing their GPA after graduating.  In particular, I believe the university maintains separate GPAs for different degree programs, so taking any post-bacc or graduate courses would not effect their bachelor GPA.