The Gate anime does cover the novels to a large extent. It takes the main plot elements from the novels, such as the opening of the gate to a fantasy world and the military's response. But there are some minor sub - plots and character developments in the novels that might not be fully explored in the anime due to time constraints or other reasons. Overall, if you've read the novels, you'll find the anime quite familiar, but it also has its own take on the story in some parts.
They often have character development that is tied to the overall story. Take Death Note as an example. Light's transformation from an initially 'righteous' killer to a power - hungry maniac is all part of the story's progression as he and L engage in their cat - and - mouse game. The story wouldn't be complete without his character arc.
The Dororo anime doesn't cover the whole manga. Some plotlines and details might be left out or adapted differently to fit the anime format and runtime.