How GnollHack differs from NetHack - hyvanmielenpelit/GnollHack GitHub Wiki
Gameplay
There are more items found in the dungeon.
GnollHack is easy in the start and hard in the end, while NetHack usually is hard in the start and easy in the end.
GnollHack is more a game of plenty, where you have many items and options. You have far more options for the ascension kit. You can also carry more in GnollHack.
It has difficulty levels appropriate for beginning players.
You can use money on several useful services. In NetHack, there are little uses for money, except for the donation in the temple.
There are plenty of identification items and services available, which means that you can identify your items easily. In NetHack, players often have to resort to price-id'ing or hope that a blessed scroll of identify identifies the whole inventory by chance.
There are fewer insta-deaths in the game compared to NetHack. Many insta-death have become delayed insta-deaths, which allow you to react to the situation for several turns.
You have plenty of information about item statistics.