No, GroupWise needs NDS to run. Even though GroupWise is not truly an NDS program, it only syncs with NDS, it still needs NDS to be administered.
GroupWise 4 was the last version of GroupWise that did not need NDS for any reason.
GroupWise 5 & 6 have Windows agents, so yes you can run GroupWise on a Windows server (and the actuall migration will be VERY easy). You will need to install eDirectory onto the Windows server. eDirectory is cross platform, and if all this server will be doing is GroupWise, then it will be the only server that MUST have eDirectory installed. To play it safe, also load a Linux or Windows box and install eDirectory onto it. This will give you a second replica of the DS database. This will also allow you to place the GWIA on a different box than the PO (recomended for security reasons). Be sure to install eDirectory onto the windows box while your current NetWare server is up and running. Get the NDS database, make the Windows box the master of ROOT, migrate mail directories over, change config files and edit NDS objects, then think about removing the NetWare server.
Remember, once you install eDirectory onto you Windows server, MS will no longer support that Windows box. Novell will, and there is a lot of info in the knowledge base when you need help (and tek-tips too).