certainly not as "Herr Gruber" unless you speak German to him
Interesting to know. I'd probably use Herr Gruber as that's what the German people I dealt with called him, even though the rest of the conversation is in English.
No, call him 'Mr Gruber' if you speak English to him. He might not mind, but he'd certainly find it odd if you used "Herr", which just means "Mister", so there's absolutely no reason to use it in another language that has a perfectly good word for it.
Interesting to know. I'd probably use Herr Gruber as that's what the German people I dealt with called him, even though the rest of the conversation is in English.