How do I implement Gmail API with a Java SpringBoot app?

Ok so firstly, I know very little about Java web programming (I've been learning Java desktop programming for 4 years) or Spring or SpringBoot. I decided I wanted to learn something new so gave myself the task of creating an email front-end that connects to gmail and retrieves the emails.

I have written the front-end in AngularJS (all new to me also but going well) and I have imported the front-end into a SpringBoot appliction created using Spring Tool Suite. I am populating the front-end with fake data from json files and I am now ready for real emails.

I have created oauth2 credentials in the Google Developers console but this is where I am stuck. I have looked at the examples on the gmail api website but I have no idea how to implement that Java when my application runs (tomcat: localhost:8080).

How do I run the Java code when the application runs. Do I use @RequestMapping or something like that?

Some steps for a novice java web programmer would be great.