<!-- By using android:spacing we can give spacing between images
     android:animationDuration="3000" -> for animation running -->
<Gallery
  android:id="@+id/languagesGallery"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_marginTop="100dp"
  android:unselectedAlpha="50"
  android:spacing="5dp"
  android:animationDuration="2000"
  android:padding="10dp" />

// get the reference of Gallery first
Gallery simpleGallery = (Gallery) findViewById(R.id.languagesGallery);

// set 3000 milliseconds for animation duration between each items of Gallery
// xml equivalent -> android:animationDuration="2000"
simpleGallery.setAnimationDuration(2000);

// set space between the items of Gallery
// xml equivalent -> android:spacing="15dp"
simpleGallery.setSpacing(15);

// set 0.25 value for the alpha of unselected items of Gallery
// xml equivalent -> android:unselectedAlpha="0.25"
simpleGallery.setUnselectedAlpha(0.25f);

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#fff"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <!-- create a ImageView and Gallery -->
    <ImageView
        android:id="@+id/imageView"
        android:layout_width="fill_parent"
        android:layout_height="200dp"
        android:scaleType="fitXY" />

    <!-- By using android:spacing we can give spacing between images
         android:animationDuration="3000" -> for animation running -->
    <Gallery
        android:id="@+id/languagesGallery"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="100dp"
        android:animationDuration="2000"
        android:padding="10dp"
        android:spacing="5dp"
        android:unselectedAlpha="50" />

</LinearLayout>

import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.Gallery;
import android.widget.ImageView;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    Gallery simpleGallery;

    // CustomizedGalleryAdapter is a java class which extends BaseAdapter
    // and implement the override methods.
    CustomizedGalleryAdapter customGalleryAdapter;
    ImageView selectedImageView;

    // To show the selected language, we need this
    // array of images, here taken 10 different kind of most popular programming languages
    int[] images = {R.drawable.python, R.drawable.javascript, R.drawable.java, R.drawable.python, R.drawable.r,
            R.drawable.python, R.drawable.javascript, R.drawable.python, R.drawable.r, R.drawable.javascript};

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Our layout is activity_main
        // get the reference of Gallery. As we are showing languages it is named as languagesGallery
        // meaningful names will be good for easier understanding
        simpleGallery = (Gallery) findViewById(R.id.languagesGallery);

        // get the reference of ImageView
        selectedImageView = (ImageView) findViewById(R.id.imageView);

        // initialize the adapter
        customGalleryAdapter = new CustomizedGalleryAdapter(getApplicationContext(), images);

        // set the adapter for gallery
        simpleGallery.setAdapter(customGalleryAdapter);

        // Let us do item click of gallery and image can be identified by its position
        simpleGallery.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                // Whichever image is clicked, that is set in the  selectedImageView
                // position will indicate the location of image
                selectedImageView.setImageResource(images[position]);
            }
        });
    }
}

import android.content.Context;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.Gallery;
import android.widget.ImageView;

public class CustomizedGalleryAdapter extends BaseAdapter {

    private Context context;
    private int[] images;

    public CustomizedGalleryAdapter(Context c, int[] images) {
        context = c;
        this.images = images;
    }

    // returns the number of images, in our example it is 10
    public int getCount() {
        return images.length;
    }

    // returns the Item  of an item, i.e. for our example we can get the image
    public Object getItem(int position) {
        return position;
    }

    // returns the ID of an item
    public long getItemId(int position) {
        return position;
    }

    // returns an ImageView view
    public View getView(int position, View convertView, ViewGroup parent) {
        // position argument will indicate the location of image
        // create a ImageView programmatically
        ImageView imageView = new ImageView(context);

        // set image in ImageView
        imageView.setImageResource(images[position]);

        // set ImageView param
        imageView.setLayoutParams(new Gallery.LayoutParams(200, 200));
        return imageView;
    }
}