As gradle continues to grow in popularity, there will be more groups utilizing different versions. When having to migrate between versions, it is very useful to know when a method/property came into existence. That is a bit difficult right now.
The javadoc @Since used for the core Java documentation is extremely useful when having to manage projects that vary across JVMs, as you often can refer to just the latest documentation and know how far back the capability can be utilized.
As there likely will be a 1.0 release coming out soon, everything could be tagged as 1.0 or left blank to assume that. Then a 1.1 etc. release would add an @Since 1.1 tag for any new properties, methods, classes, etc.