check_if_task_is_selected.gradle

import org.gradle.util.NameMatcher

def isTaskSelected(String... taskNames) {
    def matcher = new NameMatcher()
    def nameList = Arrays.asList(taskNames)
    def requestedTaskNames = gradle.startParameter.taskNames
    for (def requestedTaskName : requestedTaskNames) {
        if (matcher.find(requestedTaskName, nameList) != null) {
            return true
        }
    }
    return false
}

// Example:
isTaskSelected(":app:assembleRelease", "assembleRelease")

// Note that the result is not necessarily accurate!!!
// More information about the NameMatcher:
// https://github.com/gradle/gradle/blob/master/subprojects/core/src/test/groovy/org/gradle/util/NameMatcherTest.java

get_all_selected_tasks.gradle

import org.gradle.api.internal.GradleInternal
import org.gradle.execution.commandline.CommandLineTaskParser
import org.gradle.execution.TaskSelector

project.gradle.taskGraph.whenReady {
    def parser = ((GradleInternal) project.gradle).getServices().get(CommandLineTaskParser)
    final List<TaskSelector.TaskSelection> selections = new ArrayList<>()
    for (def taskRequest : project.gradle.startParameter.taskRequests) {
        selections.addAll(parser.parseTasks(taskRequest))
    }

    // Put your logic here, for example:
    for (def selection : selections) {
        if (selection.taskName == ":example:assembleRelease") {
            // TODO
        }
    }
}

// This result is accurate. But you can't use it in the configuration phase.