"使体质逐渐康复的" can be translated into English as "that gradually restores physical health/constitution" or "which gradually leads to the recovery of physical fitness".
Here are some example sentences incorporating this phrase:
This exercise regimen is that gradually restores physical health.
（这种锻炼方式能使体质逐渐康复。）
The therapy she received was which gradually led to the recovery of her physical fitness.
（她接受的治疗使她的体质逐渐康复。）
In these examples, the phrases effectively convey the meaning of gradually improving or restoring physical health or constitution.