From the Apple File System Programming Guide
/usr - Contains non-essential command-line binaries, libraries, header
files, and other data.
/usr/lib is for macOS installed shared libraries. Anything under
/usr/local are user-installed. So
/usr/local/lib contains shared libraries installed by you.
More likely that not, belongs to the
libgtksourceview-2.0.0.dylib gtksourceview installed by Homebrew and
libgio-2.0.0.dylib belongs to glib. The error message shows gtksourceview is compiled with a newer version of glib that is available on your system.
There are a couple of solutions, assuming you have Homebrew installed.
Reinstall gtksourceview and glib
Reinstall both software
brew reinstall glib gtksourceview
Reinstall or upgrade gtksourceview
brew upgrade --build-from-source gtksourceview
brew reinstall --build-from-source gtksourceview
If the glib dependency is out of data, it will be upgraded too. The
--build-from-source option will ensure
gtksourceview is compiled from scratch, linking with the available
glib version in your system.