'how grave' is correct and usable in written English.
You can use it to express the seriousness of a situation. For example: She was shocked by how grave the situation had become.
"That's just how grave this problem is".
No one knows better than Dudley how grave this has become for BP, Britain's largest company.
The fact that a famine has been declared shows just how grave the situation has become.
The United States should not torture prisoners in its captivity, no matter how grave their crimes.
No matter how grave or frivolous the cause, modern protesters employ far more creative tactics to draw attention.
Scholars who study tax policy in the United States dispute just how grave that injustice may be.
The question boils down to this: How grave a price would Americans pay if Afghanistan were lost to the Taliban?
There's the question not only of whether they were guilty but, if so, how grave the consequences were.
No matter how grave the danger or how minor the effort needed to prevent harm, citizens are not required to provide help.
