You can use LocaleFactory.get(Class<T extends LocalizableResource> cls) to obtain Messages, Constants or ConstantsWithLookup both on server and client side.
Add I18nFilter to web.xml or use ThreadLocalLocaleProvider.pushLocale(Locale) with ThreadLocalLocaleProvider.popLocale() to manually set thread-local locale.
Also there are a couple of additional server-only features like MessagesWithLookup interface and encoding proxies, which allow you to use localization in a more dynamic way.
Note: Unfortunately GWT-specific message formats (described here) are currently not supported except for the "static argument" format ({startBold,<b>}).
Add <inherits name="net.lightoze.gwt.i18n.GwtI18nServer"/> to your GWT XML module descriptor.
Call LocaleFactory.put(MyMessages.class, GWT.<MyMessages>create(MyMessages.class)) in your EntryPoint.
Snapshots - https://oss.sonatype.org/content/groups/public/
Releases - Maven Central
Latest release (for GWT 2.8.1):
<dependency>
<groupId>net.lightoze.gwt-i18n-server</groupId>
<artifactId>gwt-i18n-server</artifactId>
<version>0.25</version>
</dependency>
This artifact contains a copy of com.google.gwt.i18n package from GWT distribution, so you don't have to deploy the whole GWT library on the server.
In case you want only gwt-i18n-server classes, use an artifact with original classifier.