Lots of smaller shipments, each secured. Potentially air or sea (probably would do 1-10 ton air shipments for JFK-FRA). Plenty of $100mm-1b items get shipped around the world routinely, although gold is pretty uniquely fungible. It is mostly determined by insurance or willingness to self-insure, though.
This is a great article about a similar problem: http://blogs.reuters.com/felix-salmon/2011/08/23/how-to-get-...