A few oldGMCtrucks.com members, both of whom I think are also members of Stovebolt.com, posted some helpful information about steering gear lubricants. Here are links to the GM recommended steering gear lubricant and to another satisfactory product. Still, John Deere's Corn head grease is most likely the easiest-to-obtain and least expensive lubricant with appropriate flow-characteristics.

http://www.timberlinemgt.com/gmchassisgrease.aspx

http://www.hendrickson-intl.com/pdfs/trailer_PDFs/b85.pdf