Gorilla Glass is a trademark product of Corning Glass. Screen glass used by Apple is understood to be produced by Corning, but is instead to Apple’s own specification for hardness and flexibility.
Whilst iPad screens are very robust and scratch resistant, a good quality screen protector from a reputable brand is a wise investment to protect your iPad screen in some environments. A screen protector is relatively cheap to replace if damaged - unlike the very expensive screen of an iPad - the screen being the single most expensive component.
Good quality screen protectors will not degrade the screen performance, but your choice will influence the level of additional protection afforded by the screen protector. You basically have two types from which to choose - tempered glass, or plastic film.
Tempered glass offers protection from both scratches and impact. The surface is smooth - and will sometimes reduce the anti-reflection properties of the display.
Film screen protectors only protect from scratches - not impact - and often have a slightly matt appearance and texture. The textured finish reduces screen reflection - but to its detriment reduces overall screen contrast and sharpness of the image. Film protectors are favoured by some users of Apple Pencil, as the otherwise slippery surface becomes similar to using a traditional pencil on paper.
Some of the better screen protectors are manufactured by Zagg and Otterbox - the former having particularly good reputation and a lifetime replacement warranty. Zagg are often being recommend within these Support Communities.
Note that screen protectors are not themselves impervious to damage - but are a replaceable item. A damaged screen protector has likely served its purpose if damaged whilst protecting the underlying screen. Glass screen protectors are not created equally. Some have reinforced edges and/or are chemically hardened in addition to tempering.