These warnings has popped up for many compiled GWT applications for me. The threat is that hidden malicious code can exist inside the JavaScript. So, GWT is under heavy threats node due to their js compilation. There's a quite random state whether the virus signature is found or not, so i hope that the GWT group takes a discussion with the Avira team about this.
-- http://groups.google.com/group/Google-Web-Toolkit-Contributors
