You have a syntax error, you assign a value to $gethost instead of comparing.
if ($gethost === 'www.example.de') {
But apart from that, I’d do it like this, because you only want the editor-dl role in one project:
<?php
$blueprints = [];
if (Url::host() === 'select-test.test') {
$blueprints['users/editor-dl'] = __DIR__ . '/blueprints/users/editor-dl.yml';
}
Kirby::plugin('gdt/case', [
'blueprints' => $blueprints
]);