Gruber and Hans first appeared in "Card Games At 20,000 Feet", these two German brothers serve as Kaiba's elite security agents during the Battle City Tournament Finals. Gruber is in fact Roland from the original anime, while Hans is the nameless agent usually seen with him.
Other than their incredibly thick (and satirically fake) German accents and squeaky voices, they speak and behave in a manner reminiscent of stereotypical Nazis, referring to Kaiba as their "Führer" and frequently shouting, "Heil Kaiba!" Even a logo they designed for KaibaCorp appears similar to the Nazi flag.
Kaiba seems to regard them with contempt (albeit as much as he does for everyone else save Mokuba), believing them to be nothing more than "offensive stereotypes". Their names are also a reference to the villain of the first Die Hard movie, Hans Gruber, or perhaps to Captain Hans Geering and Lieutenant Gruber from 'Allo 'Allo!.
As revealed in episode 47, they are both Duelists and play it all the time; Melvin encountered them to give them a "hug". However, unlike past victims of those "hugs", they survived and continued assisting Kaiba in running the tournament in subsequent episodes. In "Yu-Gi-Oh Kai!", Hans reveals that he and Gruber aren't German at all. As of "Binary Sunseto", they use their regular voices.