Okay, What you'll wan't to do is take all of the textures used in the singleplayer inventory. You can find them by opening up the default texture pack. This is the building block for your new GUI, Everyone else is correct, you will have to build one from scratch, but you can use this as a starting point. Now you'll wan't to create a new Class that extends: InventoryEffectRenderer
Open up that class and study it. We aren't going to give you answers to such a broad problem because theres 1000 different ways to solve it, and each one would take three or four pages to explain in depth what is going on. So your job is to now Study, Study the GuiInventory, & GuiContainerCreative classes to understand how Inventory GUI's are created and maintained by the game.
Notice they all extend the above class: InventoryEffectRenderer, which means when you create your own you'll want to also extend that. These are good hints and great starting points. Now, go and do your homework
