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certainly not as "Herr Gruber"
Is it a faux pas? It's common in English to use the native form of the honorific for forrins, e.g. Tsar Nikolai and Kaiser Wilhelm, not King, Herr Hitler and Monsieur de Gaulle, not Mister.
gbj_tester-
Yes, it's a faux pas in the sort of personal conversation we're talking about (especially as most English native speakers would mispronounce "Herr"). I know that using foreign titles like that is common in English, but I haven't heard it except for the sort of prominent personalities you cite, or indeed fictional characters like the Rowan Atkinson character Mr Bean, who is usually referred to as Mr Bean, or the Jacques Tati character M. Hulot in the same way. I'd say it's much less common than in English.
In a sense, it's academic, as most Germans speak English nowadays and would generally try to switch to English when speaking to a native English speaker, so the question of how to address one in German would come up less.
Oliver Schick
German native speakers would never use the first name for a contact like that unless you'd known them for years (or you'd become friends on holiday or something like that, or you were closely related) and 'drunk to brotherhood' ("Brüderschaft trinken") with them, which means that you address each other in the familiar form ("du") rather than the polite/formal form ("Sie"). It's like tu/vous in French. Obviously, that English doesn't have grammatically-enabled polite forms doesn't mean that you can't be polite in it, it's just done in other ways. It means that there's a barrier to calling people over the age of 18 by their first names or using "du". At school, teachers used "Sie" for us from when we started sixth form, from about 16.
In the decades that I haven't lived in Germany, this seems to have become loosened somewhat in that people in trendy shops sometimes seem to want to address you as "du" even if you're clearly older, but I spend too little time there to know what the rules or indicators are for how that works.
You should address him as 'Mr Gruber' (or Dr, as the case may be) and never as Hans, and certainly not as "Herr Gruber" unless you speak German to him. I'm sure many native German-speaking businesspeople are by now very used to doing business with native English speakers and many have been addressed by first names, but it would usually jar and would certainly come across as unprofessional.