The short answer, No.
The official Gravity Forms add-ons can not be purchased individually or separately from the license they are associated with.
And before you ask, they can not be mixed and matched either.
Each Gravity Forms license type offers access to different add-ons, with the Elite license providing access to all of the add-ons.
For Example: if you need the Stripe Add-On, you must purchase either a Pro or Elite license. And if you already have a Basic license, you can upgrade to Pro to gain access to the Stripe Add-On.
To see which add-ons are available with which license type, please review the chart found on this page: https://www.gravityforms.com/pricing/
Note: You will need to scroll down to see the chart.