UPDATE: see One Android project for Eclipse and Android Studio
Currently if you create Android project in Android Studio you will have hard times opening it in Eclipse. (However opening classic Android with added build.gradle
in Android Studio should be easier)
So you can open (as general project) and then convert some modules into Eclipse project,
but still it will not be fully functional (but for a quick view inside may suffice)
from How to use Gradle to generate Eclipse and Intellij project files for Android projects
As answered in Issue 57668 by Android team (raised by @arcone)
Project Member #2 x...@android.com
The eclipse plugin is not compatible with the android plugin.
You will not be able to import an Android gradle project into Eclipse using the default Gradle support in Eclipse.
To make it work in Eclipse we will have to change the Gradle plugin for Eclipse, the same way we are modifying the Gradle support in IntelliJ
That is Android team is working on gradle plugin for IntelliJ and gradle plugin for Eclipse needs to be updated too.
What is possible with Eclipse now is
THIS IS OUTDATED, see How to import Android Studio project in Eclipse?
.1. import the project as general project
.project
<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
<name>OpenSpritz-Android</name>
<comment></comment>
<projects>
</projects>
<buildSpec>
</buildSpec>
<natures>
</natures>
</projectDescription>

.2. Put 2 Eclipse . "dot" files into modules into /OpenSpritz-Android/app/src/main
and /OpenSpritz-Android/lib/src/main

.project
<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
<name>OpenSpritz-Android-app</name>
<comment></comment>
<projects>
</projects>
<buildSpec>
<buildCommand>
<name>com.android.ide.eclipse.adt.ResourceManagerBuilder</name>
<arguments>
</arguments>
</buildCommand>
<buildCommand>
<name>com.android.ide.eclipse.adt.PreCompilerBuilder</name>
<arguments>
</arguments>
</buildCommand>
<buildCommand>
<name>org.eclipse.jdt.core.javabuilder</name>
<arguments>
</arguments>
</buildCommand>
<buildCommand>
<name>com.android.ide.eclipse.adt.ApkBuilder</name>
<arguments>
</arguments>
</buildCommand>
</buildSpec>
<natures>
<nature>com.android.ide.eclipse.adt.AndroidNature</nature>
<nature>org.eclipse.jdt.core.javanature</nature>
</natures>
</projectDescription>
.classpath
<?xml version="1.0" encoding="UTF-8"?>
<classpath>
<classpathentry kind="src" path="java"/>
<classpathentry kind="src" path="gen"/>
<classpathentry kind="con" path="com.android.ide.eclipse.adt.ANDROID_FRAMEWORK"/>
<classpathentry exported="true" kind="con" path="com.android.ide.eclipse.adt.LIBRARIES"/>
<classpathentry exported="true" kind="con" path="com.android.ide.eclipse.adt.DEPENDENCIES"/>
<classpathentry kind="output" path="bin/classes"/>
</classpath>
.3. Import as Existing Android Code into Workspace

you can then browse code in familiar way, but
even after that you won't be able to run with Eclipse ADT.
.4.
Now you can run build and tasks with gradle
CLI or Nodeclipse/Enide Gradle for Eclipse
(marketplace)

discuss at https://github.com/Nodeclipse/nodeclipse-1/issues/148
Also answered in Can't run imported gradle android project in Eclipse