Could not resolve all dependencies for configuration ':compileClasspath'. Gradle does not download dependencies

Spring-context-support version 5.0.0.M5 is not release yet.

What's more, there's no need to define a dedicated dependencies entry for spring-context-support most of the modules you specified depend on spring-context-support transitively and it will be resolved to a correct version.

Your build.gradle like this:

buildscript {
   ext {
       springBootVersion = '1.5.2.RELEASE'
   }
   repositories {
       mavenCentral()
   }
   dependencies {
       classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
   }
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'org.springframework.boot'

version = '0.0.1-SNAPSHOT'
sourceCompatibility = 1.8

repositories {
   mavenCentral()
}


dependencies {
   compile('org.springframework.boot:spring-boot-starter-data-jpa')
   compile('org.springframework.boot:spring-boot-starter-thymeleaf')
   compile('org.springframework.boot:spring-boot-starter-web')
   compile('org.springframework.boot:spring-boot-starter-security')
   compile group: 'org.hibernate', name: 'hibernate-core', version: '5.2.9.Final'
   compile('org.thymeleaf.extras:thymeleaf-extras-java8time')
   compile group: 'javax.mail', name: 'javax.mail-api', version: '1.5.6'
   compile group: 'javax.mail', name: 'mail', version: '1.4.7'
   compile('org.springframework.boot:spring-boot-starter-mail')
   runtime('org.springframework.boot:spring-boot-devtools')
   compile('mysql:mysql-connector-java')
   runtime('com.h2database:h2')
   testCompile('org.springframework.boot:spring-boot-starter-test')
}