There are a lot of captcha solutions available on web and it doesn't matter if that same captcha solution is also available to other CMS solutions in some format.
All you need is to find out exact requirements related to captcha and then do research on what solution supports your requirements. E.g.
open source vs paid/licensed one
SaaS or on-premise as a library
image based vs text based vs video based etc.
some other criteria
If I were you, I would probe further to understand what part of Google captcha is a concern - is it the brand or user interface or the complexity level etc. to narrow down on the requirements & technology piece of it.
Integrating with AEM should not be a challenge irrespective of the fact that you host it within AEM or you inject it via some Tag Management solution or some other service.