In glib-2.46.1/gio/gsettingsschema.c there's a function
"initialize_schema_sources" which is responsible for initializing the schema
sources.
It uses g_get_system_data_dirs() and traverses the result (which are supposed
to be names of directories), and it checks an environment variable
GSETTINGS_SCHEMA_DIR for the name of ONE directory; the basename of the regular
file searched for is called "gschemas.compiled".
Using
$ export
GSETTINGS_SCHEMA_DIR=/gnu/store/xz2vmk68f5jpscvpqqz1zqa7xr2ydarn-gtk+-3.20.1/share/glib-2.0/schemas
makes it work. However, that can't be the right fix. Among other things, one
can only specify a single directory that way. Also, the average user probably
doesn't even know what glib schemas are.
If that is fine anyway (why?), there seems to be already an environment
variable "GTK_PATH" set by guix, so auto-setting
$ export GSETTINGS_SCHEMA_DIR="${GTK_PATH}/../../share/glib-2.0/schemas"
should make it work for the time being.
I don't think this bug is specific to wxmaxima. I suggest to retitle it "Gtk3
FileChooser doesn't work - it doesn't find its settings" or something.
