------------------------------------------------------------------------------------
      activity_main.xml
-------------------------------------------------------------------------------------
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:text="Gallery"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/btnGallery"  />

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="500dp"
        android:id="@+id/GImageView" />

</androidx.constraintlayout.widget.ConstraintLayout>

------------------------------------------------------------------------------------
    MainActivity.kt
------------------------------------------------------------------------------------
class MainActivity : AppCompatActivity() {
    private val GALLERY = 1
    private lateinit var galleryBTN: Button

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        galleryBTN = findViewById(R.id.btnGallery)

        galleryBTN.setOnClickListener{
            val intent: Intent = Intent(Intent.ACTION_GET_CONTENT)
            intent.setType("image/*")
            startActivityForResult(intent,GALLERY)
        }
    }
}