You could use any terminal software - the old xmodem/ymodem sorts of stuff would work. The options on the two endpoints are limitless. ProTERM seems to be a good alternative on the GS end: http://lostclassics.apple2.info/downloads/?dl_cat=10

Or, if you're already connected via null modem - what about just using AppleTalk?

Edit - I see I might have answered a different question for you. Or I answered it in a different way. But, if you want to keep using ADT to move disk images as the transport medium, you can do that too. Use a tool like AppleCommander (http://applecommander.sourceforge.net) to add files to a blank .DSK image, and then transfer that .DSK in the normal way back to the GS. Oh, and you want to do this under 8.5... xx( . That won't work. Can you build the disk images using any other computer? Maybe going back to AppleTalk or terminal transfers is the way to go after all. ;-)