Plumbing code plays a predominant role, or should, in any type of modifications to an existing plumbing system, or the installation of a new plumbing system in new construction.
A gate valve is a linear type of valve that essentially blocks or allows water to move in a line, and is a perpendicular barrier to the plumbing line. A gate valve can only be open or closed; it can not be used to moderate the flow of water and is typically operated either electronically or with a knob. Gate valves are commonly used as the shutoff valves when bringing water into a structure, and may also be used as the shutoff valve on the water supply line in the structure its self.
National vs. Local Code
The only types of approved materials for gate valves are either cast iron or iron body valves. However, national plumbing code may vary from state's code, and so local codes should be adhered to when considering any kind of modifications. Most state's plumbing codes are available online. Generally, a state-specific plumbing code will be more limiting than the national plumbing code.