I would say learning and closing gaps is always part of development and must be payed. Let's say you are the most competent C++ developer in the world and have

I would say learning and closing gaps is always part of development and must be payed. Let's say you are the most competent C++ developer in the world and have the whole STD memerized (no real human is this guy I think), but part of the current project is using any other library or framework out there, then you need to dive into that documentation. And epecially as freelancer you need to learn how the companies code base every time you switch the customer or project. Another thing is if you want to qualify for certain freelance jobs. Let's say Qt is a thing you want to do professionally. Then it is very likely that you have to build up some knowledge and portfolio to get such a job. Then the freelancer pays for the time obviously.