I've uploaded my maven GWT project that I'm trying to use with GXT at: http://kjordan.net/mavengxtdesigner.zip
You'll need m2eclipse for this project.
This is the error I get, even though I've put the resources in 2 places, 1 of which is exactly the same as where it would go for a GWT Web Application Project created by Google's Eclipse plugin:
The only thing I can figure it it's looking relative to the class or something maybe seeing as there's two extra levels in this project for the src directory (main/java) that most IDEs/project types abide by, but the GWT Web Application Project doesn't (assuming GWT Designer uses only that directory layout). But seeing as I can't find a log or anything to see where GWT Designer looks for the resource files, I can't tell.
Also, using the Configure for GXT option copies the GXT jar (which I delete anyways since I'm handling it through Maven), but can't seem to copy in the resources files without an error so I do that manually.