It involve little bit modification on onItemClick(AdapterView parent, View v, int position, long id), and also onCreate(Bundle savedInstanceState).
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Gallery g = (Gallery) findViewById(R.id.gallery);
final List<String> SD = ReadSDCard();
g.setAdapter(new ImageAdapter(this, SD));
g.setOnItemClickListener(new OnItemClickListener()
{
public void onItemClick(AdapterView<?> parent,
View v, int position, long id) {
Toast.makeText(HelloGallery.this,
(CharSequence)SD.get(position),
Toast.LENGTH_LONG).show();
}
});
}