You have to differentiate GW (instantaneous power) and GWh (energy delivered in 1 hour). A 1 GW power plant generates 1 GWh per hour of operation and so on.

Your 400 million cars of 100kWh battery capacity charged once per week will require 40,000 GWh (40,000,000 MWh). A 1GW (1,000MW) power station running at 100% capacity factor will produce 168 GWh (168,000 MWh) in a week. So the absolute minimum generating capacity needed would be 238 x 1GW power stations (nuclear or otherwise).

An EV charging at home uses ~7kW (same as a reasonably high powered electric shower) so a 1GW power station can charge ~143,000 cars simultaneously and the 238 x 1GW power stations could charge 34 million simultaneously. It will take ~14 hours at 7kW to deliver 100KWh.