To put it straightly, gross negligence is the insurance term for 'it was your fault'. 

So if for example, you leave the window open and get robbed or forget to turn off the stove and come home to fire, your coverage for any resulting damage can be reduced as it could be considered 'gross negligence'. 

But fear not. With a little add-on that is quite inexpensive, you can to claim your full coverage for cases like these and protect you against your own clumsiness.