"gruelling task" is a correct and usable phrase in written English.
You can use it to refer to a task that is physically or mentally demanding, often taking a long period of time to complete. For example: "The students had to complete a gruelling task that took them three hours to finish."
Perhaps his most substantial contribution, however, was the inquiry he led and the report he wrote into the BSE crisis, what he called his "most gruelling task… by a long way".
This will also be the time where many will start the gruelling task of tallying up the horrific financial cost.
Now all that was left was the gruelling task of piercing through the ancient limestone walls of the existing cave.
David Walliams has set off on his most gruelling task yet – swimming the length of the Thames.
Mere weeks after his world title triumph at Wrestlemania 30, Bryan's father passed away, and detailing that event in the book became a gruelling task.
Pleaching is usually done to hedges and branches covering walkways and must have been a pretty gruelling task on this scale, the garden having over 60 trees.
The competition consisted of five gruelling tasks including a blind tasting, cutting precise quantities by sight and creating tasting notes for a selection of cheeses chosen by the judges.
My main concern is how gruelling a task it is going to be for those advisers staffing the phones who have to deliver the service goodness knows how many times a day, a week, a month, a year.
The downtown site, mired in ground sacred to mammon, has mixed into it a complex mulch of private rights and public responsibilities: to harmonise these competing interests in the frozen music of architecture has proved a gruelling compositional task.
Soft white caps trailing wires and electrodes measured the meditators' brain waves as they completed gruelling computerised tasks to test their powers of attention, and video recordings captured split-second changes in facial expressions as they watched images of suffering and war.
Even if it crossed the border, or was landed at the opposition's only airstrip, in Faizabad, bringing it to the front line would be a gruelling logistical task, and all but impossible when winter arrives early next month.
