图示结果如下：
可以看到Button 与Gallery的对齐方式是居中对齐，也即Button 与Parent居中对齐。
另外，
android:gravity="CENTER_VERTICAL“：Place object in the vertical center of its container, not changing its size. 这个是垂直居中对齐
android:gravity="BOTTOM”：Push object to the bottom of its container, not changing its size.放在容器的底部
android:gravity="CENTER“ ：Place the object in the center of its container in both the vertical and horizontal axis, not changing its size.放在容器的中心