GN's are aufwush grazers, i.e. they graze on the layer organic layer which covers most hard surfaces (rocks etc) in search of higher protein foods.
As someone mentioned above, they are often starved before transport in order to reduce fouling of the water - ironically to help reduce fatalitys during transport - which often causes the loss of gut flora, the healthy bacteria required for digestion.
In all honesty the fact that your fish is already in this condition, imo, means the outlook isn't good but there's still some ideas worth giving a go.
You could try adding aquarium garlic to help stimiulate it's appetite, by placing some onto other foods stuffs such as algae wafers. However, I think the best option would be to offer food stuffs in a format which is more natural to the fish. Since they're grazers, you could try the following which has been used by many a new gn owner.
Get yourself a shallow saucer, tile (an aquarium safe one) or even just a smooth rock out of your tank. Then make yourself a mixture using egg white, prawns, flake, some veg if you wish, mussel, beefheart etc and crush it into a paste - you could add the aquarium garlic to this mix also. The recipe varies depending on who you talk to but the common ingredient is the egg white as it helps bind everything together. After you've got your paste ready, 'paint' it onto the saucer or rock or whatever you're using and place it in the freezer. After it has set you can place it in the tank and hopefully the gn will find it and be more inclined to graze on it. I've also heard of people recommending using a shallow saucer and filling it with egg white and all the other food stuffs of your choice and then minimally microwaving it. This has the added bonus of 'cooking' the egg white which means you don't have the freeze it, as well as stopping the protein washing away in the water.