From Quake Wiki
The func_bossgate entity is used in Vanilla Quake to block entry to the final "Boss" level until all four item_sigils (Runes) have been previously collected by the player. It is similar to a func_wall and works in the opposite manner of the func_episodegate entity in that it removes a func_wall instead of inserting one.
Using the func_bossgate in a map:
A func_bossgate entity is attached to a brush that becomes invisible and is removed from a level only when the player is in possession of all four runes when they enter a level. As with all func_walls type entities it will not cast a shadow.
In an .fgd file there is usually this entry for the func_episodegate:
@SolidClass = func_bossgate : "Boss gate" 
This entry tells a level editor (such as Worldcraft) that the func_bossgate is to be tied to a specific brush. 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.