Add example for grid-gutter.

docs/examples/styles/grid_gutter.css

@@ -0,0 +1,11 @@
+Grid {
+    grid-size: 2 4;
+    grid-gutter: 1 2  /* (1)! */
+}
+
+Label {
+    border: round white;
+    content-align: center middle;
+    width: 100%;
+    height: 100%;
+}

docs/examples/styles/grid_gutter.py

@@ -0,0 +1,20 @@
+from textual.app import App
+from textual.containers import Grid
+from textual.widgets import Label
+
+
+class MyApp(App):
+    def compose(self):
+        yield Grid(
+            Label("1"),
+            Label("2"),
+            Label("3"),
+            Label("4"),
+            Label("5"),
+            Label("6"),
+            Label("7"),
+            Label("8"),
+        )
+
+
+app = MyApp(css_path="grid_gutter.css")