/usr/share/doc/gmt/examples/ex41/my_symbol.def is in gmt-examples 5.2.1+dfsg-3build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | # Draw a circle with a center number taken from input column
N: 1 s
0 0 1 c
0 0 0.7 $1 l
|