If by WP plugins you are referring to another form plugin, then no you don’t need both.
Akismet is auto installed on every Wordpress website. If your website is planning on having a lot of comments on blog post I’d recommend you keeping it. If not, I’d get rid of it.
The Gravity Forms Akismet add-on does require the Akismet plugin from the WordPress plugin directory to be installed. The basic Akismet integration is site wide. The GF add-on extends it with form specific settings so you can enable/disable the integration for specific forms and customize the field mappings.
The Gravity Forms Mailchimp add-on integrates Gravity Forms directly with Mailchimp so it doesn’t require any other plugins.