Gunsmoke was TV's longest running western, and TV's longest running TV show, running a full twenty years from 1955-1975, which rounded up over 600 episodes. Since there were so many episodes made, Gunsmoke is sold in three syndication packages. TV Land currently owns the third package, which is seasons 6-14 of the hour long version of Gunsmoke. (Seasons 12 - 25 over all.)

Seasons 1-6 were half hour episodes, and sometimes titled "Marshal Dillon" in syndication packages.

Seasons 7-11 are hour long episodes, filmed in Black and White

Seasons 12-20 are hour long episodes, and filmed in Color

Source: desiluweb.com/TVLand/Gunsmoke/