I do not think you can, especially since you are using a flatpak Gimp.
A flatpak is designed as "sandboxed" That is stand-alone and isolated from the user. The cursors will be in the sandbox somewhere.
For the cursors provided by MareroQ They are Windows cursors and will need converting to linux format.
see:
https://forums.linuxmint.com/viewtopic.php?t=441305Even if you convert and install to /usr/share/icons/Adwaita/cursors then you need to run a file manager as root (superuser) to copy / paste / rename (new-to-linux be very-very careful

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