Do paid Gradle plugins exist?

I think noone hinders you to do so. If you mean whether the plugin portal has some support, I don't think so.

Right, but do any exist? Has anybody done it?

https://www.code-intelligence.com/product-ci-fuzz have a Gradle plugin for their Gradle integration that is sold together with the "main" product. And which you only get via their own repository: https://docs.code-intelligence.com/get-started/setup-ci-fuzz/java/gradle#enable-ci-repository-access