I've read Reagan's stuff. Listened to a bunch of his speeches, too. He was well reasoned and sensible. That doesn't mean he had gravitas.
I like Reagan. He's one of the best Presidents we've had. It's simply that Reagan's strengths ran more towards charisma than a hard-hitting, heavyweight persona. He was more amiable than serious and severe. This is a good thing. I really don't think gravitas is what anyone needs to be a successful President.
Barry sure doesn't have gravitas, he's the epitome of empty-suit, non-accomplishment nothingness. Yet he's certainly proving effective (though not necessarily in the right way).
McCain had gravitas. He had the perfect record, a highly distinguished career in the military, a long history of diligent work in government, the on-paper perfect resume to become President. Substantive. Dignified. Serious. And it didn't help him win. If he had won, he wouldn't have been a good President.
GWB's speeches were well-reasoned and inspiring. He had a lot of great stuff to say. Thing is, nobody knows it because nobody listened to the content of W's speeches. Cheney had a lot of interesting substantive things to say, too, yet nobody listened to him either. Gravitas? W had none of it, Cheney had it aplenty, and it didn't matter a bit in either case.
Gravitas misses the point. A Prez needs to be likable, have the right policies, and be able to change peoples' minds. Gravitas doesn't enter into it anywhere.