gentranguy,
There are no conversion tools to help with the migration from mainframe GENTRAN to the NT version (that I'm aware of). I agree with hcalder in that it will be a labor-intensive task of taking each map & recreating and testing it in NT.
If you have the luxury of time in your schedule, I would recommend implementing any new EDI requests (new partners) on the NT platform first to minimize disruption with live partners. Once you have the first one implemented, you will have gained not only experience, but will also have laid the foundation for converting existing partners -- you will have resolved communications, and perhaps mapped some of the same transactions already.
After working with Harbinger, Gentran (400 & NT) and Mercator, I believe that Sterling's tech. support is far superior to the others. If you can't get the necessary training, call them for help - they know their stuff!
K. Hood