Should you use capitals for Good Morning Gentlemen?

No, you should not. The only Word that requires a capital letter is the first one, because it's the first Word in a sentence. If you used someone's name and title (Good morning, Mr. Smith; Good morning President Obama), that too would require capitalization. But the Word "gentlemen" is just a polite Word for a group of men, and is not someone's name. (It is worth noting that this Word is rarely used in conversation these days. You might still hear it during legal meetings, but only if the people there are all male; or in some countries, it is used to refer to members of the upper class. )