How do you submit a Gutenberg plugin?

On WordPress’ Detailed Plugin Guidelines page, I found a link to the Block Specific Plugin Guidelines.

I would recommend you read both before you add your plugin.

Edited to add: re: Subversion; it appears that you initially upload your plugin as a zip file. If & when your plugin is approved, WordPress will provide a Subversion repository to you, which you will be expected to use as the canonical location for plugin updates. (This doesn’t stop you from using Github or some other spot in your development workflow, but any official updates must go to your WordPress Subversion repo.)