As I understand, once the premium theme has been bought, I can use it on unlimited domains/websites for myself.
May I use the Premium Plugin also i.e. for other people when I create a website for someone else? Lets say I create a website and sell it to my customer and use my own premium GP Plugin. Is that allowed?
Your license is for unlimited websites so technically it is allowed.
However, we always suggest that the client should own a copy of a license so that one day when you no longer work for or support them, they will still have access to this support forum and receive support from us.