martinellimarco

This plugin add a new field named “Basic Captcha” in your Gravity Forms. 
It allows you to add a basic captcha that does not require access to 3rd party API. 
The captcha is a simple 4 digit combination.

GF Basic Captcha

A very simple captcha solution for Gravity Forms.

bannersky

This plugin helps you avoid spam submissions from Gravity Forms, Formidable Forms, WPForms, Contact Form 7 and Forminator.
This is the free version and you can set it up to use a blacklist. If the field value contains / same as an item / keyword in the applied blacklist, the form submission will be blocked. It’s easy to use, you just need to create a blacklist, enable settings for the form and apply the blacklist to the form fields. You can set one or more fields to validate. Validation messages can be customized for the form.
The Pro version described here: https://www.bannersky.com/gravity-forms-blacklist-and-custom-validation/ can also be used as a whitelist, IP address, email or invitation code to validate visitor input. Support for blocking email domains, giving you the option to allow or block submissions instead of always allowing submissions. Supports blocking IP addresses by country or allowing only IP addresses from a certain country.
<h3>Short Description</h3>
This plugin helps you avoid spam submissions from Gravity Forms, Formidable Forms, WPForms, Contact Form 7 and Forminator. It checks the form field content and block submitting base on your keywords. Blocking IP, Country is only supported in the Pro version.

BSK Forms Blacklist

Perfect

Checks field content and block submitting base on your keywords. Blocking IP, Country is only supported in the Pro version.

GF Basic Captcha vs BSK Forms Blacklist