The sentence "He will be greatly missed" is correct and usable in written English. You can use it when you want to express sadness about the loss or absence of someone or something. For example: We are all saddened by the news of Bob's passing. He will be greatly missed.
He will be greatly missed by many.
He will be greatly missed forever.
He will be greatly missed by all.
He will be greatly missed and fondly remembered.
He will be greatly missed by his numerous friends.
He will be greatly missed by the Syracuse community.
He will be greatly missed by the Baruch community.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com