This automatically sets your version number based on your git commit count.

version-code-as-commit-count.gradle

defaultConfig {
    # Add 10000 to move past old SVN revisions.
    versionCode gitCommitCount() + 10000 ?: 0
}

def gitCommitCount() {
    try {
        def stdout = new ByteArrayOutputStream()
        exec {
            commandLine 'git', 'rev-list', '--count', 'HEAD'
            standardOutput = stdout
        }
        def commitCount = stdout.toString().trim().toInteger()
        return commitCount
    }
    catch (ignored) {
        return 0;
    }
}

def gitRevision() {
    try {
        def stdout = new ByteArrayOutputStream()
        exec {
            commandLine 'git', 'rev-list', 'HEAD', '-n', '1'
            standardOutput = stdout
        }
        def commitRevision = stdout.toString().trim()
        return commitRevision
    }
    catch (ignored) {
        return "(unknown revision)";
    }
}