-- Option A: Import your project into Eclipse (recommended) --

Configure Eclipse following the instructions at
https://github.com/gwtproject/old_google_code_wiki/blob/master/WorkingWithMaven.wiki.md#using-maven-with-google-plugin-for-eclipse

In Eclipse, go to the File menu and choose:

  File -> Import... -> Existing Maven Projects into Workspace

  Select the directory containing this file.

  Click Finish.

You can now browse the project in Eclipse.

Now, you need to enable m2Eclipse's annotation processing functionality.
Under project properties, select Maven > Annotation Processing > Enable Project-Specific Settings,
and choose the "Automatically configure JDT APT". Click "Finish", and then right-click on the project,
and select click Maven > Update project.

To launch your web app in GWT development mode (see note below if you
have gae.home set in settings.xml):

  Go to the Run menu item and select Run -> Run as -> Web Application.

  If prompted for which directory to run from, simply select the directory
  that Eclipse defaults to.

  You can now use the built-in debugger to debug your web app in development mode.

GWT developers (those who build GWT from source) may add their
gwt-user and gwt-dev projects to this project's class path in order to
use the built-from-source version of GWT instead of the version
specified in the POM.

  Select the project in the Project explorer and select File > Properties

  Select Java Build Path and click the Projects tab

  Click Add..., select gwt-user and gwt-dev, and click OK

  Still in the Java Build Path dialog, click the Order and Export tab

  Move gwt-dev and gwt-user above Maven Dependencies

GWT developers can also use tools/scripts/maven_script.sh to push their
own GWT jars into their local maven repo.

-- Option B: Build from the command line with Maven --

If you prefer to work from the command line, you can use Maven to
build your project (http://maven.apache.org/). You will also need Java
1.7 JDK. Maven uses the supplied 'pom.xml' file which describes
exactly how to build your project. This file has been tested to work
against Maven 3.3.1. The following assumes 'mvn' is on your command
line path.

To run development mode use the Google App Engine Maven plugin and
Maven Plugin for GWT.

  mvn appengine:devserver_start
  mvn gwt:codeserver
  mvn appengine:devserver_stop

To compile your project for deployment, just type 'mvn clean package'.

For a full listing of other goals, visit:
https://tbroyer.github.io/gwt-maven-plugin/plugin-info.html
https://cloud.google.com/appengine/docs/java/tools/maven#reference_available_goals