Required or not, ground ring provides a better (lower impedance) grounding system. It really depends on what the specifications of the project call for. Here, ground rings are typically only used for sensitive electrical equipment installations - such as computer/data/network server rooms. For industrial/motor control/general light and power applications, a properly sized equipment grounding conductor (green wire), sized in relationship to the overcurrent protective device, is installed in the metallic duct and bonded to the case of each piece of equipment. Regardless, what's important is that you establish a low-impedance grounding electrode system, utilizing all available electrodes present on site - building steel, "ufer" ground (a piece of rebar stubbed out of the concrete foundation), metallic cold water, metallic gas line, etc.. All of those will bond to your "grounding electrode plate" (preferably by exothermic welds).
With a 4-pole change over switch (the better option), establish neutral-to-ground bonds at both separately derived systems - the transformer (or main switch gear) AND the generator. Bond the neutral, the equipment grounding conductor (the green wire), and the equipment case all together. DO NOT establish a neutral-to-ground bond in the changeover switch. If you do, downstream neutral-to-ground bonds will be created, causing current to flow across the metallic parts of the electrical installation. You will then install grounding conductors from the transformer (or main switch gear) and the generator to the "grounding electrode plate", establishing a connection to your grounding electrode system. Since the neutral is being switched, both the transformer and generator are considered separately derived systems and should be grounded and bonded as such.
With a 3-Pole change over switch, the generator "shares" the neutral-to-ground connection with the transformer (or main switch gear). In these cases, the neutral from the generator remains isolated from ground and is typically connected to an isolated neutral bus in the changeover switch, along with the bonded neutral from the transformer (or main switch gear). However, it's important to note that only one neutral-to-ground bond exists in these systems (not two, as in the 4-pole application).
Quick side note: Here in the U.S., building steel, ufer grounds, ground rods and rings are the preferable grounds. Bonds to metallic cold water and metallic gas are required to be made at the service entrance to the building, to ensure an effective connection to ground. However, they are considered secondary grounding methods. Nonetheless, they are still required if present on site.