The sentence "You will be greatly missed" is correct and usable in written English. You could use this sentence when saying goodbye to someone who is leaving, either permanently or for a long period of time. For example: "Goodbye, John - you will be greatly missed!".
You will be greatly missed.
You will be greatly missed by all.
You will be greatly missed, Denis.
Our Beloved Mother, you will be greatly missed.
You will be greatly missed by us all.
You will be greatly missed at our monthly Board meetings.
Cousin, you'll be greatly missed.
You'll be greatly missed Blinky :).
You brought a lot of love 2 the world & you'll be greatly missed.
I am very saddened by your enforced departure, you and Nigel will be greatly missed.
You touched so many lives and will be greatly missed by all.
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