come prendere i dati dal file csv e salvarli in grail con mysql?

Devi ottenere InputStream da MultipartFile hai superato come mostrato nella documentazione:

<g:uploadForm action="upload">
    <input type="file" name="filecsv" />
    <input type="submit" />
</g:uploadForm>

Allora;

def upload = {
    request.getFile( 'filecsv' )
          .inputStream
          .splitEachLine(',') { fields ->
        def city = new City( city: fields[0].trim(),
                             description: fields[1].trim() )

        if (city.hasErrors() || city.save(flush: true) == null) {
            log.error("Could not import domainObject  ${city.errors}")
        }

        log.debug("Importing domainObject  ${city.toString()}")
    }
}