Version 2.0 is a new major release of GNS3 which brings major architectural changes and also new features.
GNS3 was only a desktop application from the first version up to version 0.8.3. With the more recent 1.x versions, GNS3 has the possibility to use remote servers. Now, in version 2.0, multiple clients could control GNS3 at the same time, also all the “application intelligence” has been moved to the GNS3 server.
Version 1.3 is a new major release of GNS3 which brings major architectural changes increasing stability and easy to use.