The orginal Gramps creators are not around, and there is nowhere a definition of how this old xml looks like.
So, if you have a lot of data there you want to convert, you should:
1/obtain the old linux genealogy live cd version 1.0 http://ftp5.gwdg.de/pub/linux/gramps/
2/run it, and use it to convert the old gramps to a new one you then store on usb. The xml of Gramps 2.0 or 2.2 will import in Gramps 3.2
2/install or obtain one of the images of a linux distribution of end 2005, begin 2006
3/install gramps inside of it via our old sourceforge packages
4/convert the old xml to one of the official documented one.
The above is actually so easy that I don't think any developer will try to write a converter for the non-documented xml formats