It’s kind of a grey area, as it could be seen as re-selling GP Premium to your clients with no kick back to me.
However, it’s 100% GPL, so you’re free to do whatever you like.
In cases like this, it’s really appreciated if a license is purchased for each individual client, as they will have GP Premium and updates for life.
Also, if they have their own license, they can request that an account be created so they can take advantage of my help in the forums here.
WP Show Posts is a little different as you would have to renew your developer license each year, so I still get a little kick back.
Hope this helps 🙂