The Eclipse XML editor works nicely with GSPs. It's not a GSP editor, but it does a few nice things:

• Command+Shift+F formats
• Opening tag completion for html elements, and closing tag completion for any element including taglib elements
  
• Syntax highlighting
• Structure error markers
  

It doesn't however work out of the box. To get it working:

1. Window->Preferences
2. Search for 'Content Types'. In the list navigate to Text->XML
3. Add '*.gsp' to the file associations list

That's it - happy GSP editing. I wonder how difficult it would be to extend the XML editor to be Groovy/Grails aware?