Yeah that's true, Gasol didn't come over to the NBA until he was 24. I love Kessler and am very high on him, he is scoring at a blistering 70+% inside right now. Giannis has proven that sometimes trying to develop weaker areas can not be worth it if he can further hone aspects you are elite at. I guess I just don't think Marc Gasol is the right comp for him, but if he can develop some of those areas that would be amazing.
FT% does seem to be a pretty indicator of shooting potential, and Kessler's 55% is pretty rough at this point. Gasol shot 73% as a rookie and got up into the 80%'s. But Kessler seems to have a good drive and work ethic. He proved in college that he has the willingness to shoot outside the paint and if he can keep working on that he can be exciting. Karl Malone shot 48% from the line his rookie season and got it up to almost 80% by the end of his career, and developed a nice rounded offense game to say the least, but comparing his progress to a top 20 player of all time is dangerous haha.