Quarkus Gradle plugin is not yet published to the Gradle Plugin Portal, so you need to add the following at the top of your './settings.gradle' file:

pluginManagement {
    repositories {
        mavenCentral()
        gradlePluginPortal()
    }
    resolutionStrategy {
        eachPlugin {
            if (requested.id.id == 'io.quarkus') {
                useModule("io.quarkus:quarkus-gradle-plugin:${requested.version}")
            }
        }
    }
}

Or, if you use the Gradle Kotlin DSL, you need to add the following at the top of your './settings.gradle.kts' file:

pluginManagement {
    repositories {
        mavenCentral()
        gradlePluginPortal()
    }
    resolutionStrategy {
        eachPlugin {
            if (requested.id.id == "io.quarkus") {
                useModule("io.quarkus:quarkus-gradle-plugin:${requested.version}")
            }
        }
    }
}

This won’t be necessary anymore once the Quarkus Gradle plugin is published in the Gradle plugin portal.