adding codegym to my resume

I'm not sure it's worthwhile to add it until you've completed it. A prospective employer won't know offhand what "level 16" translates to in terms of your knowledge, and they likely won't spend time reading through the lessons on the website to figure that out, not when they have other applicants. I'm not sure I'd feel comfortable applying to anything until completing CodeGym's full course, otherwise there are too many holes in your knowledge that are going to cause issues, and they're going to want to hire someone who doesn't need additional training if they can. An exception, of course, would be if you were applying for an internship position, since it's expected that you're in the learning process when you take one of those on. In that case, I would list it under Education, something like "Enrolled in online course at CodeGym" and then in bulleted points under that, list some of the most recent skills you have learned rather than the level (which they still won't understand). For example, I've just completed Level 3 of Java Core, so I might list: Interfaces & Abstract Classes, Inheritance, Polymorphism, Arrays/Maps/Lists, and basic loop logic as things I've learned so far. I think that will make it clear to the employer what you know, far more than saying "CodeGym Level 13".