I found this hack which includes a myriad of changes to the original. There are a few in particular (see below) that I would want to apply to my hack as well. So I'm wondering how can I apply only the desired changes to my ROM?
GFF is pretty extensive, and lots of people want to use it as a base for their hack. Though, AFAIK Grond didn't set up any kind of segmentation -- it's just all one big hack.
So if you want to separate it, you'll have to "hack the hack", so to speak.
I'd take one of the below paths:
1) I think there are some IPS patchers that show you individual changes the patch makes, and allows you to select which sections you want to patch. I know I've used a patcher like this before, but I can't for the life of me remember the name of it. Anyway, if you find such a patcher, you could use some trial-and-error to apply different sections of the hack until you've isolated what parts do what. Or... you can contrast the patch changes to the disassembly (or a ROM map) to see if you can guess what parts of the patch might be changing before applying them.
2) Given the popularity of GFF, it's possible someone has already done this. Try googling for a GFF ROM map, or a list of GFF changes.
3) A lot (but not all) of these changes have been done independently (and individually) by other hackers. Rather than using GFF in bulk, you could take those individual hacks. There's a big post on gamefaqs about common bugfix/enhancement patches. You might find some stuff there. https://www.gamefaqs.com/boards/522595-final-fantasy/45575058
That's about all the advice I can give you. I don't really care enough about GFF to bother actually looking at it in detail.
Though again... GFF is huge
so pretty much any approach you take here is going to be daunting and time-consuming.