Is it possible to open an emailclient such as gmail when I click a button in my app?
You can simply use below code when for no attachment:
Intent i = new Intent(Intent.ACTION_SENDTO);
i.setData(Uri.parse("mailto:support@mailname.com"));
i.putExtra(Intent.EXTRA_SUBJECT, "Feedback/Support");
startActivity(Intent.createChooser(emailIntent, "Send feedback"));
For details I recommend to visit: https://developer.android.com/guide/components/intents-common.html#Email