"a $1.4 billion gap" read as "a 1.4 billion dollar gap"? Or "a 1.4 billion dollars gap"?

"a 1.4 billion dollar gap"...

a 1.4 billion dollar gap"...

Yeah, I agree, although I don't pretend to know much about rules of grammar, etc. You're talking about the gap, and it's "size," not the dollars themselves (which merely modify your reference to the "gap"). Gap is singular, even though the amount of dollars, standing alone, is plural.