GIMP through 2.10.2 makes g_get_tmp_dir calls to establish temporary filenames, which may result in a filename that already exists, as demonstrated by the gimp_write_and_read_file function in app/tests/test-xcf.c. This might be leveraged by attackers to overwrite files or read file content that was intended to be private.
Gentoo Security Scout
Base on information available here:
Indeed this is just a unit test run when a dev/packager/other runs make check on the source code. This is not part of GIMP at all and nothing is installed with this code.
I added a comment there to clarify: https://gitlab.gnome.org/GNOME/gimp/issues/1689#note_254032
and reading through the thread. This is not an issue of Gimp.