There is something about using g_autoptr() together with G_DEFINE_AUTOPTR_CLEANUP_FUNC() when using different GLib versions which I don't understand (this affects also the other g_auto... variants and its G_DEFINEs).
The documentation says
"The way to clean up the type must have been defined using the macro G_DEFINE_AUTOPTR_CLEANUP_FUNC()"
When using for example GLib 2.62 and using the G_DEFINE macro this results in errors like
/usr/include/glib-2.0/glib/gmacros.h:1032:49: error: redefinition of ‘glib_slistautoptr_cleanup_GtkTreePath’
1032 | #define _GLIB_AUTOPTR_SLIST_FUNC_NAME(TypeName) glib_slistautoptr_cleanup_##TypeName
Leaving out the G_DEFINE macro will solve the problem and the program works just fine.
However, on older GLib versions like 2.50 (which is for example still used by Debian 9), using the G_DEFINE macro will not result in an error message. But I can't see any changes reflected by the GLib documentation. I cannot determine when exactly the aforementioned change of behaviour has been introduced. How am I supposed to cope with this when I want to support all GLib versions from 2.50 on?
The issue is probably in Gtk, not GLib. g_autoptr() was supported for most things in Gtk+ 3.22 (the one in Debian 9) already: so you shouldn't have to call G_DEFINE_AUTOPTR_CLEANUP_FUNC() on Gtk types yourself. GtkTreePath however was still missing a call: this was added in 3.24, see https://gitlab.gnome.org/GNOME/gtk/-/commit/86dd1e37a70e9bae057a9a11332f7254cda242e8.
You'll probably have to do the macro call behind a version check if you want to use g_autoptr() with TreePath on Gtk < 3.24.