I'm trying to chroot with the GParted LiveCD onto an existing 64-bit system, attempting to fix grub.
I've never found one. I use clonezilla as a rescue CD (and network-boot image), and that DOES have a 64-bit version.
If you want 64-bit gparted live, you'll have to make your own, just replacing the kernel with a 64-bit kernel will be enough - it will be able to run the 32-bit userland on the gparted ISO.
BTW, when you do manage to chroot into your system you will need to bind-mount /dev, /sys, /proc (and optionally /dev/pts) into the chroot environment so that grub-install will have access to the info it needs to re-install into the MBR.
If your target system is mounted under /target, something like this will do the job:
for i in dev proc sys dev/pts ; do mount -o bind /$i /target/$i ; done
I have my netbootable clonezilla image configured to set that up as an alias called 'prepare-chroot-target' so I don't have to remember it :)