/usr/share/plplot5.13.0/cmap1_gray.pal is in libplplot15 5.13.0+dfsg-6ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | v2 rgb
2
0.0 0.0 0.0 0.0 1.0 0
1.0 1.0 1.0 1.0 1.0 0
|