I know a fair bit about amazon and there is little threat to GXL.
1. Pet food is not their thing and even if it is, perishable food is not that easy with the online model
even amazon has to open a physical store, amazon go for food.
2. Electronic and General merchandise is their target, pet stuff would be somewhere at the bottom of their priority if ever
3. Once GXL build a large retail network with key locations, it will acts as natural blocker for new comer
due to Australian large landscape with 23 odd million people so moving large bulking pet food around cost a lot of money so there is little value buying pet food online without the physical locations.
GXL will be the bunnings of pet care shop once it reaches that stage, other can have a crack at them but once they secure all the good site with first mover advantage it be a bloody hard battle.
Masters is a textbook example of how a business failed with bad location
and the established dominant player with key locations