The absolute first step is to have a gmail account. I don't think I need to help you with that but after that things get a bit more complicated.
Syndesis is using the OAuth Standard to connect to the GMail API. On the Settings section on Syndesis it requires you to enter a Consumer Secret and Consumer Key (see Figure 1)
Figure 1. Syndesis GMail Settings
In order to obtain access to the GMail API you first need create a new project at [https://console.developers.google.com].
Figure 2. Select Project
Figure 3. Create Project SyndesisQS
Figure 4. API Services
Figure 5. Enable GMail API
Figure 6. Create Credentials
Figure 7. Click 'Configure consent screen'
Figure 8. Configure consent screen
Add the Application Name and Support Email, and make sure to add the nip.io as an Authorized Domain!
Figure 9. Configure Web App and Callback URL
Figure 10. Copy from Client Credentials Popup
Figure 11. Paste Client Credentials into Syndesis
You can now create a GMail Connection and these credentials will be used.