Function gtk_source_gutter_lines_get_line_extent 

pub unsafe extern "C" fn gtk_source_gutter_lines_get_line_extent(
    lines: *mut GtkSourceGutterLines,
    line: c_uint,
    mode: GtkSourceGutterRendererAlignmentMode,
    y: *mut c_double,
    height: *mut c_double,
)

Available on crate feature v5_18 only.