应该是修饰marched,副词一般都是修饰动词或形容词的,几乎不会用来修饰介词或名词.
其实仔细一看,这句话有问题：outside可作为介词或名词,这句话中outside后跟了of说明是当名词用,但是前面又没有跟at,所以是有问题的.要么改成The striking workers marched slowly and deliberately outside the locked front gates of the company.或The striking workers marched slowly and deliberately at the outside of the locked front gates of the company.