It sounds like your Angular app is rendering the login page, using something like the Okta SignIn Widget? If that is the case you want to look into the SPA/implicit flow.
However if you want to use a code flow (and still have a custom login page) you can do this at the zuul gateway application: https://github.com/okta/samples-java-spring/tree/master/custom-login.
(though I’d suggest starting with the basic redirect flow until you have it working: https://github.com/okta/samples-java-spring/tree/master/okta-hosted-login).
At that point you can use an access token between your applications (assuming you use the @ResourceServer annotation, and things should just work) IIRC Zuul just passes the same access token down stream.
I’m still not 100% sure we are talking about the same thing, sorry in advance, but let us know!