Actually, this is quite simple. But a few people asked me because they had some issues when installing the GWPA plugin in eclipse. So here is the way which works for me without any problem.
The version you are choosing (32bit or 64bit) has to match your installed Java version (see step 1).
Start eclipse and choose "Help" > "Install new software"
Use URL to load plugins: https://tools.hana.ondemand.com/juno Then select only the component "Toolkit for Integration Gateway (GWPA, Developer Edition)" and click on Next.
eclipse will automatically find out the Dependencies which are required for this component, confirm this and start the installation.
Accept the upcoming permission message and restart eclipse after successful installation.
In eclipse open Window > Preferences > OData Development > Integration Gateway and specify the address of your SMP server plus admin port (default 8083) with user and password. Click on "Test Connection" button to verify that your SMP server can be reached and that the provided user and password are correct.
Then you can start using the "Integration Gateway Toolkit" by choosing File > New > Other... > OData Development
You finished installing the GWPA plugin and can start your Service Implementation Project now...