/usr/lib/x86_64-linux-gnu/pkgconfig/gl.pc is in libgl1-mesa-dev 8.0.4-0ubuntu0.6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | prefix=/usr
exec_prefix=${prefix}
libdir=/usr/lib/x86_64-linux-gnu
includedir=/usr/include
Name: gl
Description: Mesa OpenGL library
Requires.private: x11 xext
Version: 8.0.4
Libs: -L${libdir} -lGL
Libs.private: -lm -lpthread
Cflags: -I${includedir}
glx_tls: no
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