I have a simple form with two checkbox type fields, each one is populated by a custom taxonomy. Both fields are marked as "required".
When I test the form, I check some options in the two lists of checkboxes, and on validation it gives me the error : "This field is required."
See screenshot for an example
EDIT : I'm on Gravity Forms 22.214.171.124 and Gravity Forms Custom Post Stypes 2.0