It's a process of (gradual) (progress).
It's a process of (gradual) (development).
It's a process of (gradual) (evolution).
这里只能用gradual
任选,供参为什么不能填develop gradually?1.a process of () ()，结构决定了其后中心词、即第2个（）必须是名词，由此逆推、则第1个（）只有形容词——gradual2.develop gradually，这里说不过去，语法上不知所云