If you’re getting bounce backs from gmail, then you know it’s not easy to find your way though Google’s website to track down a form to get delisted.
I have spent a long time going their site, answering questions on forms to finally bring me to the following URL:
https://support.google.com/mail/contact/msgdelivery
Fill this form out, and within about a week, your email should start delivering to gmail accounts again. Typically Google will not write you back, they will just lift the block.
***NOTE
Before you fill in the form, make sure that your server is no longer sending out spam. Otherwise, they will either reject you, or blacklist you again right away.