The relationship to GDP is meaningless. The relationship to the budget is not.
Currently the interest cost is $365 billion yearly. That's huge, it's half the defense spending. That's double what it was ten years ago. It is projected to be over $800 billion in ten more years. That's also 1/3rd, more or less ( currently) of all discretionary spending. More than any other component other than defense. In fact if you take the next five categories, Housing, Vets, Education, and Government, they don't even come close to that. You have to add in almost ALL discretionary spending outside of defense to equal the amount spent for interest.
That's not healthy and it's not sustainable. Whatever lies they spin about GDP and interest don't matter. Ten years from now we simply won't have the money to pay the military salaries and retirements without huge increases, social security spending will be much higher, and interest will more than double. All that in an economy growing at 2-3% if lucky.
Oops, why not viable long term? Well, first, it hasn't worked well for decades. We have had some serious financial problems and a Great Recession. We're deeply in debt from that recession and no way of paying it off, the interest payments are only going to grow. It's a bit like credit cards. If you are clever, you can live off them for years. Then you run out of available new cards to spread the debt to. I've seen that happen to friends living in Piedmont, with $80k in CC debt. Nobody argues that debt is good or bad. Only that when it reaches a point where a large proportion of your budget is servicing it, you are starting to have a large problem. When the prognosis is for that debt to grow over 100% in ten years, you have a huge problem. When major corporations have been issuing bonds as fast as they can, many of them basically junk, and municipalities issuing uncovered bonds as well, you begin to have a massive problem. The US is swimming in debt; personal, corporate, state, and Federal. It won't magically disappear because some arbitrary number called the GDP reflects some odd statistic.