Is it possible to open an emailclient such as gmail when I click a button in my app?
Yes. You can launch it via Intents.
Intent i = new Intent(Intent.ACTION_SEND);
i.putExtra(android.content.Intent.EXTRA_EMAIL, new String[]{ emailAddress });
i.putExtra(android.content.Intent.EXTRA_SUBJECT, subject);
i.putExtra(android.content.Intent.EXTRA_TEXT, text);
startActivity(Intent.createChooser(i, "Send email"));