Is there standard way in grails to manage plurals in international strings from property files?

You can use an embedded ChoiceFormat in the i18n message. Using an example from the java.text.MessageFormat Javadocs you could define something like:

numfiles.message = "There {0,choice,0#are no files|1#is one file|1<are {0,number,integer} files}."

Then in your GSP:

<g:message code="numfiles.message" args="[numfiles]"/>

Where numfiles is an integer value.