Lots of topics from the Gravuty Forms community forum show up when searching Google for a variation on gform.addAction is not a function (like this one), so I figured I'd publish a solution that worked in my particular case. TL;DR I think the team should take another look at the admin script dependencies.
Note that this applies to version 2.9.10 of the plugin, it is possible that a future version may fix this specific issue (I hope).
The form edit screen loads a gforms_hooks.js file and inserts it as an inline script attached to the gform_gravityforms script enqueue with id gform_gravityforms-js-before. This script begins with if ( ! gform ) { ..., so if gform has been defined in the window scope already this script will not add the addAction and other functions that additional scripts rely on. However, at least in my case, a script with id gform_gravityforms_libraries is loaded