How can I override a property defined in build.gradle?

Check whether the project has a property, and if not, set it to the default value:

ext {
    if (!project.hasProperty('MY_NAME')) {
        MY_NAME = 'John Doe'
    }
}

See: https://docs.gradle.org/current/userguide/build_environment.html#sub:checking_for_project_properties

If you need to do this for multiple properties, you can define a function:

def setPropertyDefaultValueIfNotCustomized(propertyName, defaultValue) {
    if (!project.hasProperty(propertyName)) {
        ext[propertyName] = defaultValue
    }
}

ext {
    setPropertyDefaultValueIfNotCustomized('MY_NAME', 'John Doe')
}