ASME B30.2-2005 will give some general guidelines for top running gantry cranes. It is pretty general and doesn't give lots of design specifics. When you get into more detail design, many engineers take requirements from CMAA 70 & 74 for bridge cranes and use them in design of gantry cranes. There is also ASME NOG-1 which covers bridge and gantry cranes for nuclear facilities. ASME NOG-1 tends to be overkill for most uses.
