From Quake Wiki
The func_bossgate is a solid brush entity that will spawn on level load, unless player has all the item_sigils (Runes). 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.
A func_bossgate entity is attached to the brush(es). Like func_wall it also supports animated textures and switches between them when targeted. It can also be killtargeted and also contolled with skill level appearance spawnflags.
As with all func_wall type entities it will not cast a shadow.
In Vanilla Quake func_bossgate is used in the start map to block entrance leading to the ending level with the finall "Boss". When player completes all four episodes and collects four item_sigils (Runes), the floor would be removed once player enters start map again.
|targetname||The targetname of the brush entity.|
|256||Not on Easy||The Entity will not spawn on Easy difficulty.|
|512||Not on Normal||The Entity will not spawn on Normal difficulty.|
|1024||Not on Hard||The Entity will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||The Entity will not spawn in Deathmatch mode.|
@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.
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED func_bossgate (0 .5 .8) ? This bmodel appears unless players have all of the episode sigils. */