I know this is a pretty nooby question, but how would I do it, here is my code

 

if(guibutton.id==12){
		authenticatePin(enteredPin);
	}
}

private void authenticatePin (String pinCode) {
	EntityClientPlayerMP player = Minecraft.getMinecraft().thePlayer;
    	if (pinCode.equals(ItemDebitCard.checkCardPin(player))) {
    			System.out.println("Correct Pin!");
    			player.openGui(FinancialMod.instance, 1, world, x, y, z)
    		} else {
    			System.out.println("They Dont MATCH!");
    		}
}

I can't be sure about this as I've never played around with GUI elements before (something I'm about to get into). Have you tried calling FMLNetworkHandler.openGui() from the button click listener though?

 

EDIT: I didn't really look though your code. Interesting that we both have different GUI calling methods. You'll probably have to find a way to close your current GUI, which I don't know how to do, so I'll shut up now...