Can they have careers, as medical doctors, etc. And can they accomplish lots in life? Or is life worthless for them at this point?
Yes, they can become successful.
I am an individual with a GAD and am also bipolar. Life as a person with these problems is 100 times easier than it was even 20 years ago. These are accepted as genuine mental conditions that need treatment, psychiatrists are open to helping individuals with GADs through therapy, and there are now reliable medications available that have minimal side-effects (and there are multiple medications that can be tried, in case one or a combination of meds don’t provide the desired results). It is understood that individuals with these conditions are still human, still have feelings and thoughts and valid emotions and concerns that need to be addressed, and it’s easier to function on a day-to-day basis without having to first explain to the world – be it romantic interests, friends, family, or employers – that we have "a problem" that may make us unstable or difficult – if not impossible – to work with.

So, yes, individuals with GADs can indeed succeed in today’s world, and have. You’d probably be surprised how many well-rounded, educated, intelligent individuals have some kind of GAD or other issue.