The fore end of the Grizz is a bit larger than an 870, so as noted it will need fitting, or, if you want, you can replace the 'pump' with a Remington one. Also the stock may have a bit of a bump where it bolts to the receiver. Nothing that would affect function, just something that bugs some folks.
Synthetic or wooden mounting parts are called stocks. There are various grips that may be part of the butt stock, or separate. You may even refer to the fore stock, or forend.
The term 'furniture' hearkens back to the era when the lock(s) and barrel(s) were attached by various means to the stock. Furniture means the visible small metal parts, such as the trigger guard, barrel bands, escutcheons, top lever, safety tab, forend release lever, etc. Typically, these were made of steel or brass.



























