I am trying to lock content on my Wordpress site, but obviously I want Google to crawl it. When I lock the content, the header comes back as a 302 redirect(redirecting to a page where someone can login or sign up). Will google still index it as if it were 200?
If users are redirected to the login page then so will Google, so no, the page will not be indexed.
The login page (the page you are redirected to) will be indexed instead, unless you have a noindex robots meta tag (or equivalent HTTP response header) on the login page (which you probably should have anyway).
At the end of the day Google only wants to index content that is publicly available for all to see.