The two-week wilderness survival training was absolutely gruelling.

gruelling 是形容词，来自动词 gruel（原指"用粥折磨"，引申为"使精疲力竭"），加上后缀 -ing 构成形容词，表示"极其艰苦的、令人筋疲力尽的"。这个词特别强调体力和精神上的极度消耗。