I don't believe the field declaration you've provided could lead to the error message you've quoted.
Please provide a fully-compilable class (including all import statements) and a list of the maven deps we need to build it, and we'll dig into this error to try and help you through the problem.
Yes you're right, that error was for something else. After doing a mvn clean, I managed to get GXT 3.0 form fields, TextField, etc. to work with Errai, and even the GXT validation is working. Cool.