As far as I am aware there are two problems, each with a partial solution:
GDL bitmaps can be thrown around as pictures each with a different origin and direction, which is what the Garden Works trees do to produce those leafy branches. Artlantis handles them as wallpapers, so you have to deal with a pattern. My partial solution is to create the texture in Artlantis by overlaying three textures made by repeating the ArchiBAM image with different spacing and different orientations, using the Invisible shader and masking out the white in the pictures. This is a partial solution because the resulting Artlantis trees are not nearly as realistic as what you originally got in ArchiCAD --they are more of a clearly abstract representation, although you get volume and shade and transparence and you can tell a birch from a pine. Another problem is that Artlantis 4.5 starts producing some funny rainbow shadows when working with overlayed transparent textures, which is something that didn't happen in previous versions.
The other thing is that (I think this started with AC 8 ) the leaf and bark materials, which are coded into the ArchiBAM trees, get exported with a different name each time you modify the model. The Artlantis material gets a name like 'GDL-45-SilverFir leaf', say, which for exactly the same tree becomes 'GDL-58-etc.' after you made some changes to the landscaping or added some other object with GDL-coded materials and re-exported (so that 'open with reference' will not help for the leaf material). My partial solution for this was to get into the GDL code, remove the material code, create new materials ('SilverFir_leaf') I carry around in my attributes all the time, and make an ArchiCAD module with all the trees I use for option-clicking and inserting, and an Artlantis file made out of that module, with all the texture work, I use for importing the tree materials from any model. This is a partial solution because the dumber tree objects will show OK in Artlantis but, having lost their origin and orientation stuff, now look very bad in ArchiCAD.
The results are definitely not realistic but OK for sketch presentations:
http://www.ar-in.com/samples/SGate06_closed.jpg
http://www.ar-in.com/samples/040525NCourt3.jpg
