I would have to say no. Grails competes with Rails (yeah, it's the Groovy knock off). Seam is an integration framework, Grails is a full blown web development environment. They use some of the same technologies, but they don't really compete. I'd actually love to see Grails use something besides JSP in the view layer.
Both Seam and Grails can be used to develop web apps -- but so is every other JEE component technology. Grails is more of a MVC framework. In the world of Seam, JSF is the MVC framework. Seam itself does two things:
1. Provide the "Glue" to link JSF with other JEE components (e.g., EJBs, other OSS frameworks such as jBPM, Drools, Quartz, iText, and even Groovy).
2. Extend and enhance JSF with more state management/ dependency injection / security / Restful and other features. A lot of Seam's enhancement has since made its way into JSF 2.
Of course, this is just a simplified view of things. Seam now supports non-JSF web frameworks such as Tapestry and Wicket -- it does the same #1 and #2 for those frameworks as well.