Gallery: Add margins to the gallery block using theme.json (#4631)

* Gallery: Add margins to the gallery block using theme.json

* Remove unused code

blockbase/theme.json

@@ -396,6 +396,13 @@
 					"fontFamily": "monospace"
 				}
 			},
+			"core/gallery": {
+				"spacing": {
+					"margin": {
+						"bottom": "var(--wp--custom--gap--vertical)"
+					}
+				}
+			},
 			"core/navigation": {
 				"typography": {
 					"fontSize": "var(--wp--preset--font-size--normal)"