How are some gradle dependencies working with no version supplied

It is worth mentioning that the trick is called BOM (bill of materials) and the actual versions can be checked in the related POM file (in this example, it is for the version 2.7.0) inside spring-boot-dependencies package. This is mentioned in the Spring Boot official documentation here: Build Systems.

Another way that Spring provides this (for non Boot projects) is through Spring Platform BOM where it actually provides version for the following dependencies.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'io.spring.gradle:dependency-management-plugin:0.6.0.RELEASE'
    }
}

apply plugin: 'io.spring.dependency-management'

dependencyManagement {
    imports {
        mavenBom 'io.spring.platform:platform-bom:Athens-SR2'
    }
}