Call another task from a task in gradle

As suggested one method would be to add a finalizer for the task

task beta << {
    println 'Hello from beta'
}

task alpha << {
    println "Hello from alpha"
}

// some condition
if (project.hasProperty("doBeta")) {
    alpha.finalizedBy beta
}

Then we can execute the other task if needed. As for executing tasks from another tasks you cannot do that. Task declaration is declarative not imperative. So a task can depend on another task but they cannot execute another task.

$ gradle -q alpha
Hello from alpha
$ gradle -q alpha -PdoBeta
Hello from alpha
Hello from beta

You can use

a.dependsOn 'b'

Or

a.dependsOn b

Or

task a(type: Exec, dependsOn: 'b') { ... }

etc

See adding dependencies to tasks

To summarize and combine the answers from @JBirdVegas and @lance-java, using non-deprecated doLast instead of leftShift (<<):

task beta {
    doLast {
        println 'Hello from beta'
    }
}

task alpha {
    doLast {
        println 'Hello from alpha'
    }
}

// some condition
if (project.hasProperty('doBeta')) {
    alpha.finalizedBy beta // run 'beta' after 'alpha'
    // or
    // alpha.dependsOn beta // run 'beta' before 'alpha'
}