Intent URI to launch Gmail App

The best way to do it, is by using the generic way/method:

Intent intent = new Intent(Intent.ACTION_SEND);
intent.setType("plain/text");
intent.putExtra(Intent.EXTRA_SUBJECT, "Email Subject goes here");
intent.putExtra(Intent.EXTRA_TEXT, "Your Message goes here");
startActivity(Intent.createChooser(intent, ""));

This will give the users a choice where they can pick GMail (if installed) or any other email supporting app they have.

startActivity(new Intent(Intent.ACTION_SENDTO, Uri.parse("mailto:"+mailId)));

Working as for me is just simple like this:

 Intent(Intent.ACTION_SEND).apply{
            setPackage("com.google.android.gm")
            type = "text/plain"
            putExtra(Intent.EXTRA_TEXT, "Go, go share text!")
        }.also{readyIntent->
            startActivity(readyIntent)
        }

This snippet will open a chooser which is supposed to point to Gmail inbox.

val intent = Intent(Intent.ACTION_MAIN)
intent.addCategory(Intent.CATEGORY_APP_EMAIL)
try {
    startActivity(Intent.createChooser(intent, getString(R.string.open_email_app)))
            } catch (e: ActivityNotFoundException) {
    showErrorDialog(R.string.error_activity_is_not_found)
            }

Neither was working for me, as the subject and message were always empty, until I found the official solution here.

fun composeEmail(addresses: Array<String>, subject: String, attachment: Uri) {
    val intent = Intent(Intent.ACTION_SEND).apply {
        type = "*/*"
        putExtra(Intent.EXTRA_EMAIL, addresses)
        putExtra(Intent.EXTRA_SUBJECT, subject)
        putExtra(Intent.EXTRA_STREAM, attachment)
    }
    if (intent.resolveActivity(packageManager) != null) {
        startActivity(intent)
    }
}

This answer is old,but still appears in the Google Search at first position.

Hence,from the android documentation,the better way to do this now is :

public void composeEmail(String[] addresses, String subject, Uri attachment) {
Intent intent = new Intent(Intent.ACTION_SEND);
intent.setType("*/*");
intent.putExtra(Intent.EXTRA_EMAIL, addresses);
intent.putExtra(Intent.EXTRA_SUBJECT, subject);
intent.putExtra(Intent.EXTRA_STREAM, attachment);
if (intent.resolveActivity(getPackageManager()) != null) {
    startActivity(intent);
}

}

More information can be found out at here

final String package = "com.google.android.gm";
// return true if gmail is installed
boolean isGmailInstalled = isAppInstalled(MainActivity.this, package);

Intent intent = new Intent(Intent.ACTION_SEND);
intent.putExtra(Intent.EXTRA_EMAIL, new String[] {"mail@gamil.com"});
if (isGmailInstalled) {
    intent.setType("text/html");
    intent.setPackage(package);
    startActivity(intent);
} else {  // allow user to choose a different app to send email with
    intent.setType("message/rfc822");
    startActivity(Intent.createChooser(intent, "choose an email client"));
}

// Method to check if app is installed
private boolean isAppInstalled(Context context, String packageName) {
    try {
        context.getPackageManager().getApplicationInfo(packageName, 0);
        return true;
    } catch (PackageManager.NameNotFoundException e) {
        return false;
    }
}

Intent URI to launch Gmail App

Intent mailClient = new Intent(Intent.ACTION_VIEW);
mailClient.setClassName("com.google.android.gm", "com.google.android.gm.ConversationListActivityGmail");
startActivity(mailClient);