All of the design drawings I've seen over the years show gate valves as full bore design.  That is, with the gate fully open nothing protrudes into the flow.  There may be some that I haven't seen the drawings on that have a reduced flow area, but I've never heard of them.
One caveat, when people back a valve off the back seat they sometimes get too enthusiastic, and sometimes people get trash on the stem and think the valve is open when it is partially closed.  Neither of these problems result in a measurable pressure drop across a gate valve, but they are both really hard on a smart pig.
Gate valves that are piggable are called thru-conduit or slab gate valves. The design ensures that a pig will pass through safely without getting stuck.
Pigging is the act of causing a mechanical device (a "pig") to traverse a flow line for the purposes of either shifting liquids trapped in a gas line, cleaning debris from any flow line, or separating products in a liquid line.

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