CKEditor (what this plugin uses as the editor) comes with a spell checking (SCAYT) … I haven’t really used it though and looking at it now it’s not as good as the what the browser provides.
If I recall right, the creators of CKEditor are working on a major update that improves this.
If you want to see what I mean, have a look at my demo page:
https://demo.itsupportguides.com/gravity-forms-wysiwyg-ckeditor/multi-page-form-ajax-enabled/?gform_debug
In my quick tests I noticed that ‘tesssst’ wouldnt coming up as an error in Google Chrome but it would in Firefox – which is odd because they should be using the same dictionary service. (which makes me wonder – how do you choose the language of the dictionary) .
All settings for this plugin can be found in the WP-Admin under the Forms -> Settings -> CKEditor menu.
(NOTE: I need to push out an update to automatically enable SCAYT when enabled from the settings page)