the IGuiHander IMPL
https://github.com/loordgek/Extragenarators/blob/trytofix/src/main/java/loordgek/extragenarators/GuiHander.java#L42
the guiID is used for opening the correct gui see ^
the EntityPlayer::openGui
https://github.com/loordgek/Extragenarators/blob/trytofix/src/main/java/loordgek/extragenarators/blocks/BlockGenBase.java#L75
and the gui
https://github.com/loordgek/Extragenarators/blob/trytofix/src/main/java/loordgek/extragenarators/client/gui/GuiFurnaceGen.java#L11
does this help you ?