I know Google Web Toolkit (GWT) is a development toolkit for building and optimizing complex browser-based applications. GWT is used by many products at Google, including Google Wave and Google AdWords. It's open source, completely free, and used by thousands of developers around the world.
It can be integrated in java based web applications....
Is there any suitable replacement of GWT for asp.net web application? If so,what is it?
Consider SmartGWT. It has a built-in REST connector that is easy to connect to REST services on the .NET platform, which Visual Studio can help you generate.
On the .NET platform, the free open source (LGPL) edition is all you need. If you need commercial license terms, those are available too.
Take a look at Script#, but I'm not sure if it's still in development. "Essentially the Script# compiler is a C# compiler that generates Javascript instead of IL."
http://www.nikhilk.net/Entry.aspx?id=121
If you're just looking to hook up an ASP.net web application with GWT, try
GWT is platform agnostic. It can be easily integrated with any web application, not just Java. You just tell it to inject widgets into elements on the host page - the host page could be static HTML.
The built in RPC mechanism makes it easy to do RPC with a Java on the server side, but you can certainly use JSON or XML to exchange data with your server. It's a little more work, but not impossible. Look up 'overlay types' - these make working with JSON data extremely easy.