How can I remove News Articles from the sidebar menu, but keep Gallery and Pages in the menu?

You didn't say what theme you are using.

If you are using the [i]zenpage[/i] theme, the easiest is probably to use CSS.
In [i]style.css[/i] add the following:
.menu:first-child {display: none;}

Or comment it out on the sidebar.php of the theme (if it has one).