I looked at this awhile ago gave my self a headache and decide I probable could not afford the licenses anyway.
This is what I discovered.
Daz 3d does support exporting as FBX whether it is compatible with GG import function I do not Know.
If you want to use a model from DAZ in any games engine you will need to have either the developer license from the models creator or the models Interactive license You need licenses not only for the base model but also any clothing props textures etc you may choice to add.
(Read the terms of each license carefully some Model creators add addition conditions to the standard terms. )
The Models will have to be decimated quite heavily to fit GGs requirements. Also if the option is available you should remove hidden polygons particularly when you add clothes etc (both options are available in Posers development versions not sure about daz3d)
As for rigging and animation IN THEORY you should be able to use Daz3s animation system and export them in the FBX provided GG FBX is supporting animations correctly. remember to make an not of the begin and end frames of each sequence you will need them in order to use them in GG (note you will also require the correct licenses for the animations) .
A lot of this depends on what options the DAZ3d FBX exporter supports and whether GG also supports output from them.
One area you will defiantly have a lot of problems is textures DAZ models use a lot of textures you need to be able to turn them into atlases (this may be an option in the FBX export you will have to check that out )
All this is provisional on GG importer supporting DAZ3d version of FBX + animations extra.
As the FBX import function appears to be still under development you will be aiming at a moving target.
Good luck you'll need it.
“Everything should be made as simple as possible, but not simpler.”
Albert Einstein