we are trying to set up Sparkpost in CiviCRM, but we are using G-Suite and can't use DNS? So tried to use another method, but when trying to add either Postmaster or Abuse G-suite says those are reserved. What am I doing wrong?
Your question does not say why you can't use DNS?
Google Apps will I think let you configure the abuse@ and postmaster@ addresses as aliases (Google refers to these as "groups"). Google will still process those addresses as well, but I think you can configure them so you can field the Sparkpost setup email.
Here is Sparkpost docs on that: https://www.sparkpost.com/docs/tech-resources/abuse-postmaster-google-apps/