From Quake Wiki
The func_episodegate is used in Vanilla Quake to create a special func_wall in the start level. It is tied to a specific item_sigil (Rune) in the players possession upon entering a level. In the game it is used to block access to an episode that has already been completed by the player. As such there are four different func_episodegates each one related to a specific item_sigil.
Using the func_episodegate in a map:
A func_episodegate is attached to a brush that becomes visible and solid only when the player is in possession of a specific rune picked up in an earlier level. As with all func_walls it will not cast a shadow.
In an .fgd file there is usually this entry for the func_episodegate:
@SolidClass = func_episodegate : "Episode Gate" [ spawnflags(Flags) = [ 1 : "Episode 1" : 1 2 : "Episode 2" : 0 4 : "Episode 3" : 0 8 : "Episode 4" : 0 ] ]
This entry tells a level editor (such as Worldcraft) that the func_episodegate is to be tied to a specific item_sigil. Episode 1 is the default here. The lack of an Appearflag indicates that there is no option for selecting a skill level (for the obvious reason that you would normally want this entity to effect all skill levels). There is no rule against adding that to a custom .fgd file if you wanted to and that could lead to some interesting design choices for a level creator.
See also: func_bossgate