DRV8323: question from layout guide

Part Number:DRV8323

Hi, 

I am checking layout from the guide line and having some problem: 

1. why the width of gate signal needed to be more than 20 mils? what will be the problem if not?

2. what is the mean by route as a differential? 

does that mean the length need to be the same of SLx and SPx?

thank you.