Try adding android:layout_marginTop="?android:attr/actionBarSize" to the parent of your GridView, or the GridView itself if it doesn't have one.
This will push your layout down so that it rests below the ActionBar.
Edit
You may want to conside using a ListView instead of a GridView. Reason being, you can easily achieve that effect by creating a fake header and then calling ListView.addHeaderView. You can't do the same with a GridView. What you're talking about can definitely be done with a GridView, but it will require you to subclass it and modify it quite a bit.
Header
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingTop="?android:attr/actionBarSize" />