If memory serves, the GWT Designer tool was never part of GWT itself, but was a component that could be installed in Eclipse, allowing for WYSIWYG UI design (in part by running something like dev mode while you were editing?). Changes that the author of the project made were reflected in standard source files (.java and .ui.xml) that GWT itself was able to compile - as such, a newer compiler version can likely still compile (with small or minimal changes to source) such an old project. I would suggest small steps in upgrading, to keep such changes (if any) manageable.
If you're hoping to run GWT Designer itself, you probably need a very old version of eclipse - my memory here is that it wasn't used enough to be worth maintaining. The code still lives at
https://github.com/gwt-plugins/gwt-designer, but I can't guess what it would take to bring it back to life. I have a memory of this being based on WindowBuilder, and some internal changes in WindowBuilder requiring substantial changes to the gwt-designer project, which never was done.