Something like this

tasks {
   "jar"(Jar::class) {
       manifest {
         attributes 'Implementation-Title': project.name
       }
    }
}

Thanks for reply. I tried this but seem to have had no effect:

"jar"(Jar::class) {
        manifest {
            attributes(mutableMapOf(
                "Implementation-Title" to project.name
            ))
        }
    }

Could this have anything to do with it being a Spring Boot JAR?

Yeah, correct, but be careful with

withType(Jar::class.java)

you set this attribute for all tasks with type Jar, your original groovy snipped did this only for task with name “jar”, but you right, if Spring Boot have own Jar task this is the way to solve your problem, or set this to specific task