Many of the same things affecting lifespan are the same things that cause age-related degeneration. Any medical improvement to lifespan will almost certainly be an improvement to health. (There are non-medical things that could improve average lifespan without substantially improving average health, such as reducing causes of fatal accidents, but those wouldn't improve maximum lifespan, only average lifespan.)

And yes, of course it's a gain. You could always choose not to take those extra years, if you really don't want them. It's good to have that possibility available.