Can i add a app to gparted iso?

I think you can mount an ISO image under your running system and make changes. I found a couple of guides that might be helpful:

How to Mount an ISO File on Linux

Edit and repack .iso bootable image

I haven't done this before, but I think you can chroot into the mount directory, and run package manager commands in the mounted image to install another package.

Or I have an alternative suggestion that might or might not be easier. I've been hearing a lot about immutable/atomic distros, and people designing their own images. You could make your own ublue image, for example, with whatever you want on it.

A promising looking starting point is github:ublue-os/startingpoint. Ignore the "Installation" instructions, and follow the "ISO" instructions instead.

Or I saw recently an announcement of a new way to build atomic images that is supposed to be easier than ever, BlueBuild

Read your distro documentation on customizing image.

You can reduce the space taken by /home and give that to /? Just make sure to reduce only the free space, or else you will lose data. And it also probably depends on the type of filesystem, I guess?