Gen dec is the General Declaration. Has the info about the flight, like passenger count, crew count, etc and requires signatures.

Crew dec is the crew decleration that each crew member has to show to customs along with your passport when reentering the country. Usually the FA's have a big packet of customs and immigration forms for both the pax and crew. They usually just call you up and ask if want them so you can fill it out.

Unless you have a bunch of purchased items to delcare, it takes about 30 seconds to fill it out. More than once I've done it waiting in line at customs.