Since Gravity Forms 2.0 the new “no captcha” reCAPTCHA API is included in Gravity Forms. This was a highly requested feature and works pretty fine for most users, but sometimes specially if you’re using old reCAPTCHA keys or the reCAPTCHA authentication in Gravity Forms settings was done using previous API version you can get one of the following pretty cryptic message in your reCAPTCHA field:
ERROR for site owner: Stoken disabled or Error for site owner: Stoken expired