Are you just looking for a partition editing tool? Rescuezilla ships with gparted and some other backup/rescue tools and boots from live usb though gnome disks isnt included if i remember. Gparted distributes its own live boot utility. You could also look into making a persistent live installation of ubuntu or other distro and have those installed and theyll remain on the disk.

Edit: Rescuezilla comes with gnome Disks so this may be what youre looking for

you could use MXlinux and it's remaster tools/feature to build your own custom USB/iso.

MXlinux makes such a task rather easy.

remove stuff you don't need to get it as small as you desire.

Live Kit may be what you are looking for. Haven’t tried this myself, but it looks straight forward. You basically turn an already installed system into a live distro.

Fedora has both.

you could install your favorite distro with all your customizations and needed programs on a flash drive and use that as a rescue system

Solus Budgie Edition.

I need it preinstalled to be independent of internet access.