The "gaprop" tag is a special tag introduced in Galactic Adventures.
Any "props" made for an adventure (walls, keys, backdrops, characters, decorations, etc.) can be tagged with "gaprop" before it is published to prevent the creation from apearing in the game in any slot. Many of the Maxis made creations that come with GA have this tag.
Many users have mentioned glitches with the gaprop tag not working properly, with tagged buildings or space empires in their games. The tag is also known to not work at all in the Civilization stage.
The gaprop tag does not work with other tags. You must put gaprop by itself in the tags.