What I did:

1. In application.properties added

    application.config.admin-role-name=ADMIN

2. In build.gradle added

dependencies {
    ...
    annotationProcessor "org.springframework.boot:spring-boot-configuration-processor"
}

3. In *Application.java file annotated the *Application class with

@ConfigurationPropertiesScan

4. Annotated one of my @Configuration classes with

@EnableConfigurationProperties(ApplicationStartupConfig.class)

5. Created the following ApplicationStartupConfig class:

@Component
@ConfigurationProperties(prefix = "application.config")
public class ApplicationStartupConfig {    
    private String adminRoleName;    
    public String getAdminRoleName() {
        return adminRoleName;
    }
    public void setAdminRoleName(String adminRoleName) {
        this.adminRoleName = adminRoleName;
    }
}

6. Finally, cleaned with gradle, built with gradle and did IDE’s Refresh Gradle Project

Unfortunately, even though ApplicationStartupConfig is successfully populated on startup (I can use it via @Autowired in my ApplicationRunner code), Eclipse says:
‘application.config.admin-role-name’ is an unknown property. application.properties