How to add gridview setOnItemClickListener

If you want to get text that clicked item you can use this

gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                String item = ((TextView)view.findViewById(R.id.ID_OF_TEXTVIEW)).getText().toString();
                Toast.makeText(showMissions.this, "" + item, Toast.LENGTH_SHORT).show();

            }
        });

You are requesting click on the item(not on the button inside the item) so need to change your child XML layout

from

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
>

<Button
    android:id="@+id/city"
    android:layout_width="183dp"
    android:layout_height="90dp"
    android:textSize="19sp"
    android:textStyle="bold"
    android:text="Code\n\nCity"
    android:gravity="center"
    android:background="@drawable/city_btn_tablet" />

</RelativeLayout>

to

<?xml version="1.0" encoding="utf-8"?>
<Button
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/city"
    android:layout_width="183dp"
    android:layout_height="90dp"
    android:textSize="19sp"
    android:clickable="true"
    android:textStyle="bold"
    android:text="Code\n\nCity"
    android:gravity="center"
    android:background="@drawable/city_btn_tablet" />

or you can add ClickListener inside adapter

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

    View gridView;

    gridView =new View(mContext);

    gridView = inflater.inflate(R.layout.gridview_members, null);

    Button city = (Button) gridView.findViewById(R.id.city);

                city.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

    //Toast here

        }});

    return gridView;
}

GridView is clickable no need to put button inside GridView.

To add listener to the grid add the following code:

// Implement On Item click listener
gridView1.setOnItemClickListener(new OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

        switch (position) {
            case 0: 
                break;
            case 1:
                break;

        }
    }
}); 

And so on you can add all numbers in the cases.