Any additional undergrad after graduation counts just the same as "more senior year" in your undergrad GPA. Anybody can raise their GPA by taking more undergrad - even after graduation. Many of us who did this went ahead and just earned a second bachelor's, because why not - when we're taking extra classes in a different discipline.
Anything graduate - even a SMP - is not undergrad. So this will not go into your undergrad, but it will go into a graduate GPA, which is not considered in your BCPM or Amcas overall gpa.
What I did - graduated in 2002 in another area. Went back to undergrad as a second-degree seeking student (anybody can choose this on their FAFSA whether or not they complete the degree or not), got financial aid for tuition, earned a second degree in microbiology. Taking the pre-reqs only would still have had me applying when I did, so taking the few extra courses to get a second degree did not delay when I could apply to school and go. It was a great decision for me. Showed a great record of handling heavy science classes, and I applied with a strong GPA.
I hope this helps. I'm not sure why people always suggest SMP right away, when sometimes the people asking for suggestions really just need a record of handling hard sciences and earning A's at the undergrad level so it improves their BCPM and overall undergrad AMCAS GPA.