I have done but they said they won't send out stickers or sticker up a non-Gray Nicolls bat. So i'm really unsure as how to get them.
The short answer is, you can't.
GN do the most work to protect their IP, it amazes me that any brand would be anything but extremely vigilant and strict about their labels/branding being available publicly, as there is the potential for genuine stickers to end up on ropey bats made cheaply in Pakistan and then sold as genuine.
Unless you send a bat to them for them to apply the stickers, you won't get any.
You may very occasionally see a genuine set on ebay that have been acquired via an unscrupulous sponsored player or former sponsored player, but thats it.