To subscribe to feeds, you'll need either support in your client (Lagrange has support, so does deedum, I think amfora does, but I'm not sure), or you'll have to install and run feed aggregating software like CAPCOM, gmisub, or comitium. CAPCOM subscribes to atom xml feeds, gmisub subscribes to the simple gemtext feed spec, and comitium subscribes to both. All of those output a gemtext file that you can either read locally, or put on a Gemini server to open from anywhere.
I kind of considered this to be outside the scope of the quickstart; I might add an extra page.
As for logging in to things – some sites let you log in with a client certificate, though what you can do with that is limited by the simple nature of Gemini input. It's not really suitable for e.g., being able to write entries on the Pub.
Oh, so I can subscribe with any RSS reader?