а разве там не
android:layout_gravity="fill_horizontal"

http://developer.android.com/reference/android/widget/GridLayout.html

авторLimitations
GridLayout does not provide support for the principle of weight, as defined in weight. In general, it is not therefore possible to configure a GridLayout to distribute excess space between multiple components.

Some common use-cases may nevertheless be accommodated as follows. To place equal amounts of space around a component in a cell group; use CENTER alignment (or gravity). For complete control over excess space distribution in a row or column; use a LinearLayout subview to hold the components in the associated cell group. When using either of these techniques, bear in mind that cell groups may be defined to overlap.


и почему именно gridlayout ?
может можно обойтись и с relativelayout ?