The personality psychologists I know tell me that Martin E. P. Seligman, the originator of the idea of "learned helplessness" and the author of helpful books such as The Optimistic Child, advised his student Angela Duckworth to call "grit" by that term as a marketing move to make people pay more attention to the idea. In the psychology literature, people have been writing for a long time about the personality dimension "conscientiousness," and grit is very little distinguishable from that. I'm a homeschooling parent of four children, and it's plain as day to me (and consistent with all the statistics in the research literature) that conscientiousness matters at the margins. Two equally smart children (per IQ test), one conscientious, one less conscientious, will differ in life outcomes. Everyone knows that.

The HUGE question that is well worth investigating is how much parenting or good teaching or anything else can change a young learner's level of conscientiousness. Conscientiousness changes over the course of life, so it isn't completely fixed for anyone. It varies (somewhat) among identical twins, so environmental influences do matter in development of a given degree of conscientiousness. But so far we don't know a lot yet about what kind of parenting or what kind of teaching might work best to develop conscientiousness. That's well worth learning about. Calling it "grit" is regrettable because that prompts people to neglect the prior research literature.

AFTER EDIT: Comments below this comment asked for definitions of "conscientiousness." I'll put a few links here.

http://pages.uoregon.edu/sanjay/bigfive.html

http://http-server.carleton.ca/~tpychyl/011382000/BigFive.ht...

http://www.personalityresearch.org/papers/popkins.html