"使体质逐渐康复的" can be translated into English as "gradually restoring physical health" or "contributing to the gradual recovery of physical constitution".
Here are two example sentences incorporating these translations:
1、 Regular exercise is gradually restoring my physical health after the long illness.
（长期锻炼正使我在大病之后体质逐渐康复。）
2、 A balanced diet and adequate rest are contributing to the gradual recovery of my physical constitution.
（均衡的饮食和充足的休息正使我的体质逐渐康复。）