How to make scrollable tab in Android?

Simply just put app:tabMode="scrollable" inside the xml.

Eg:

<android.support.design.widget.TabLayout
    android:id="@+id/tabs"
    android:background="@color/colorPrimary"
    app:tabMode="scrollable"
    app:tabTextColor="#ccc"
    app:tabIndicatorColor="@android:color/white"
    app:tabSelectedTextColor="@android:color/white"
    android:layout_width="match_parent"
    android:layout_height="40dp" />

I tried to set tab gravity to MODE_SCROLLABLE

That is not a gravity value. That is a value to be passed to setTabMode() on the TabLayout (tabs.setTabMode(TabLayout.MODE_SCROLLABLE)).

This sample project from this book has an activity that toggles between three fixed tabs and ten scrollable tabs:

/***
  Copyright (c) 2012-15 CommonsWare, LLC
  Licensed under the Apache License, Version 2.0 (the "License"); you may not
  use this file except in compliance with the License. You may obtain a copy
  of the License at http://www.apache.org/licenses/LICENSE-2.0. Unless required
  by applicable law or agreed to in writing, software distributed under the
  License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
  OF ANY KIND, either express or implied. See the License for the specific
  language governing permissions and limitations under the License.

  From _The Busy Coder's Guide to Android Development_
    https://commonsware.com/Android
 */

package com.commonsware.android.tablayout;

import android.os.Bundle;
import android.support.design.widget.TabLayout;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity {
  private SampleAdapter adapter;
  private TabLayout tabs;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    ViewPager pager=(ViewPager)findViewById(R.id.pager);

    adapter=new SampleAdapter(this, getSupportFragmentManager());
    pager.setAdapter(adapter);

    tabs=(TabLayout)findViewById(R.id.tabs);
    tabs.setupWithViewPager(pager);
    tabs.setTabMode(TabLayout.MODE_FIXED);
  }

  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.actions, menu);

    return(super.onCreateOptionsMenu(menu));
  }

  @Override
  public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId()==R.id.fixed) {
      item.setChecked(!item.isChecked());

      if (item.isChecked()) {
        adapter.setPageCount(3);
        tabs.setTabMode(TabLayout.MODE_FIXED);
      }
      else {
        adapter.setPageCount(10);
        tabs.setTabMode(TabLayout.MODE_SCROLLABLE);
      }

      adapter.notifyDataSetChanged();
      tabs.setTabsFromPagerAdapter(adapter);

      return(true);
    }

    return(super.onOptionsItemSelected(item));
  }
}