Game Development Stack Exchange is a question and answer site for professional and independent game developers. It's 100% free, no registration required.
In your main ApplicationListener where you create a camera object there will also be a resize method. If not override/implement it. It takes two arguments and when the window is resized these will be the dimensions of the newly resized window. Just set the cameras size to the new window size and call camera.update(); Should sort it! (I assume you are using the built in OrthographicCamera class of Libgdx)