“Having a gun cannot prove or disprove anything for you. Coward or brave, depends on what you fight with, not what you hold with.”

这明显是中式英语，因为Having sth不是人，无法证明或者证伪什么东西。因为只有人才有思维，才能证明什么。prove还可以是一个有形的物体证明什么，而不是having sth动名词这个动作状态来证明什么，如果那样的话也应该是A gun in hand 比having a gun要更符合逻辑些。另外coward depends on也是很怪的说法。

应该是“A man having a gun cannot prove or disporve anything for him.  Being coward or brave depends on what you fight against in your mind, not on what you hold in your hands." 或者是：

"A gun in hand cannot prove or disprove anything for you.  To be coward or brave depends on what you fight against in mind, not on what you hold in hand."

最后那个在depends前面的逗号也很莫名其妙。

职老说的对，英语好不好，在于你的中式思维而不是英语语法和用词。当然英语语法也是英语的思维的表现之一，但不是全部。