Although there is no groovy-all-2.5.6.jar published, you can still depend on org.codehaus.groovy:groovy-all:2.5.6, which resolves to a POM file that lists the individual component jar files as dependencies.
Please clarify exactly what you’re doing such as Gradle version, the dependencies in your project, the error you’re seeing, etc. In the simplest case, declaring a two dependencies, one on org.codehaus.groovy:groovy-all:2.5.6, and another dependency that depends on org.codehaus.groovy:groovy-all:2.4.15, there is no problem resolving Groovy 2.5.6 despite the lack of groovy-all-2.5.6.jar.