Is there a way to recover edits to an entry before I installed the Entry Revisions plugin?

No, there is not an easy way to recover edits that were made before installing the Entry Revisions plugin. The Entry Revisions plugin saves the entry each time it is changed. Without a plugin saving those changes, there is no "change log" for Gravity Forms entries.

If you have a website backup plugin (or your web host provides automated backups), you can download a backup of your database. Then, you could copy the database rows for the entry, and enter the prior data in manually.