'How greatly' is correct and usable in written English in certain contexts.
For example: "How greatly increased demand for the product has influenced sales figures."
How greatly will precision medicine change cancer treatment?
How greatly will Peyton Manning be affected by his sore ankle?
How greatly Dickens personally had changed appears in remarks by friends who met him again, after many years, during the American reading tour in 1867 68.
How greatly was his confidence our gain!
How greatly the transport costs differ between different transportation options is affected by several uncertainties such as price trends for fuel.
After his father's death, it dawned on him just how greatly he had failed.
It's nostalgic and simple, and highlights how greatly a short love story can impact its lovers.
But too few doctors, experts say, understand how slowly methadone is metabolized and how greatly patients differ in their responses.
O ye men who think or say that I am malevolent, stubborn or misanthropic, how greatly do you wrong me.
It is a singular and delicious quirk of baseball, how greatly one player can affect expectations from game to game.
With the group favourites, Belgium and Italy, lying in wait, both Ireland and Sweden know how greatly a victory would enhance their prospects.
