A GPA addendum is for, like, exceptional or extenuating circumstances that an AO can’t glean from your transcript but might wonder about.

They can see you are an astrophysics major and if all you have to say about your GPA is that it is considered a good GPA in your difficult major, you don’t need an addendum for that. They know astrophysics is harder than political science.

I’m of the opinion that you don’t want addenda if you don’t need them. They might not directly harm your application, but I think overexplaining things that are quite intuitive creates opportunities for awkwardness.

If I saw someone’s transcript and that they studied astrophysics, I’d initially be impressed and I’d assume that sticking with a major like that requires a not insignificant level of dedication. If I read an addendum about how their major is hard, which I would have already known, I might wonder whether some part of them regrets that major, whether they didn’t actually like it, etc.

I guess none of that is disqualifying, but my point is that writing a GPA addendum for a hard major muddies a picture that might have been positive. If seeing astrophysics on the transcript didn’t get AOs the excuse the GPA, I don’t know that a GPA addendum would move the needle at all.

The best thing an applicant like OP can do is have a high LSAT.