Gradle helper functions for loading properties file and merging into project properties

build.gradle

loadExternalProperties('version.properties')

def loadExternalProperties(fileName) {
    def Properties props = new Properties()
    file(fileName).withInputStream { stream ->
        props.load stream
        props.each { prop ->
            project.ext[prop.key] = prop.value
        }
        stream.close()
    }
}

version.properties

buildVersion=1
versionString=1.0.0

This would allow you to use project.buildVersion to access the values.