I have a Gravely L from c.1955 in my shop right now. They are well made machines and one of my favorites! Your Gravely was made in 1966; the official model is a C 7.6.
Old old L's (like mine) did not come with governors. They were an added cost option, but a bit nice when you have to do some work. They are a bit hard to find today reasonably; those who have them keep them just because they make your life that much better while in use!
Pierce governors aren't too bad to work with if you are familiar with centrifugal governors.
Check all your springs and connections to make sure they are working as they are a bit tricky to get correct, like all linkages, if you are not familiar with them. I had an early L retrofitted with a Pierce that had binding linkages. This PDF is very helpful in this area:
As John mentions in the PDF, probably the weights are a little tight if it runs like an airplane.
Gene Gull rebuilds them, as well as Rick Mathis of the Gravely Shop: http://www.gravelyshop.com/
Ben W. in VA