By default Gaia will spawn your trees using the tree system that comes with the terrain system.


The upside of this is that it will do LOD's etc, the downside is that you can not run any scripts.


To get around this you need to set them up as prefabs and then spawn them as Game Objects.


You can learn more about this here : http://www.procedural-worlds.com/gaia/tutorials/spawners-gameobject-poi/