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.
Courses taken through different programs generate separate GPAs. When somebody (like a graduate admissions committee member) is looking at your undergraduate-level grades, they are going to pay primary attention to the grades for courses you took at your primary undergraduate institution (or institutions, if you transferred during your regular undergrad career). They are probably not going to average your grades from different places, particularly if it is obvious that you took additional courses (through an easier program) after you finished a normal undergraduate degree.
However, at many institutions, faculty members may change grades after a course has been completed. The rules for how long they have to make changes are highly variable; it could be limited to within two years, or five years, or subject to no time limit at all. This is probably not going to be very helpful to you though. Grade changes are rare, and faculty are very unlikely to raise your grades for courses you took in the past. It would take a lot more than just convincing them that you are better student now than you were then (which would be difficult in itself).