Render FieldLabel initially as false, ui behavior shows/hides. If initial rendering of fieldLabel is setVisible(true) everything is fine. If initial rendering of FieldLabel is setVisible(false) then the grid will NEVER render.
I've committed a fix to SVN for this issue as r2787. This change will be included in the next release.
For now, you can work around this problem by creating an appearance for FieldSet that has a style method that includes the GXT FieldSet CSS as well as your own CSS that sets the .body element to have the style rule width:100%.