Allow me to describe some of what happens in the background when an application is sent to us. I direct a graduate school in a Russel Group university in the UK.
What you describe is a common technique in applications to our programmes. Applicants wish to draw attention to a perceived deficiency by supplying a perspective that mitigates or explains away the deficiency. "My overall GPA is
x, but for core courses is it
y >> x."
The fact of the matter is this: in my graduate school, you are ranked first by cumulative GPA. If you do not make the cut-point, then your application is assessed no further. Those that pass this hurdle move on to the next stage. We very rarely segment GPA into groups of courses (although most students and some academics think that we do). Stating that your GPA is a subset of courses is greater than the cumulative GPA is problematic for us for two reasons: (1) it emphasises that your performance is inconsistent and (2) that performance may be in areas that are important.
Let's take a recent example that was put to me by a master's programme director. The applicant's cumulative GPA was 3.7/4.0, but in her math modules in the last two years of her degree, her GPA was 3.9/4.0. The master's programme director wanted permission to accept the student on these grounds. On the basis of cumulative GPA, the student was ranked 45th our of 64 applicants. We had space for only 25 new students.
When we looked at the non-math courses, we noted that the student's main weaknesses were in computer science-related courses as well as in English for academic purposes. The picture that this student was giving us was that of a highly competent math person who was weak in computers and had difficulty expressing themselves in a ways expected of an academic.
Clearly, the applicant was not in the top 25. In fact, there were 19 others ranked higher than her on the ordered list. How would you be able to justify accepting this student in this context?
I strongly suggest that you provide evidence in your personal statement that you are a worthy candidate. GPA is only one part of that consideration. In fact, it is the easiest to determine because it exists in the transcript. Spending precious real estate in your personal statement trying to explain away a poor GPA by offering different combinations of courses that will produce a better GPA is not very wise, in my view. I want you to show me that you are a great fit for our programme.
Good luck to you.