You can now export layer geometries from gINT for use in PLAXIS 3D 2010 or later.
There is documentation in gINT's Help.
Also, there is an instructional video available on the Geotechnical WIKI under Videos on gINT Topics. You can also download the example files used in the video to follow along here.
Once exported, data can be brought into PLAXIS 3D and used for modeling. Only the layer geometries are exported, so you need to assign layer properties in PLAXIS. There are additional constraints of use that are detailed in the video and in gINT's Help.