Wordpress - Upload folder is not writable, even when permissions are correct

That's a server permission problem. According to the WP docs:

Any file that needs write access from WordPress should be owned or group-owned by the user account used by the WordPress (which may be different than the server account). For example, you may have a user account that lets you FTP files back and forth to your server, but your server itself may run using a separate user, in a separate usergroup, such as dhapache or nobody. If WordPress is running as the FTP account, that account needs to have write access, i.e., be the owner of the files, or belong to a group that has write access. In the latter case, that would mean permissions are set more permissively than default (for example, 775 rather than 755 for folders, and 664 instead of 644).

From http://codex.wordpress.org/Changing_File_Permissions

How can I troubleshoot this issue?

Ask Webfaction. Sounds like they need to give you the ability to chmod files/folders or they need to do it for you.

And, FYI, search first for a common issue like permissions; there are many answers already on WP Dev.

Upload folder is not writable. Export and file upload features will not be functional.

This specific error string comes from Gravity Forms and not WordPress itself.

Go to Settings → Media and check the value for "Store uploads in this folder" is correct. Using the default of wp-content/uploads will remove the error as long as that directory has the correct permissions set.