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).
You could serve different content to Google than you do for normal users. ie. let Google see the real content while redirecting normal users. This is called cloaking. However, this considered black hat these days and could see your site severely penalised.
Alternatively, allow everyone to see your site for a month (or so) and allow Google to index the content in the usual way and then lock the content on your site. However, your indexed pages are likely to drop off after this time, but hopefully users will have had a chance to find it.
Or only publish the first part of the article and have the full article available to authenticated users. Google might be able to pick up keywords and subject matter from the publicly available teaser.
At the end of the day Google only wants to index content that is publicly available for all to see.