I'm pretty sure you should run 'gradlew build' in the cmd (Just like you ran 'gradlew setupDecompWorkspace' when initializing the workspace for the mod). It should export the mod automatically as a .jar file into the eclipse folder, in one of the sub-directories there (Don't know on the top of my head).
I'm not sure how to solve the gui problem, but the difference between onItemUse and onItemRightClick is (I believe) that the first one is whenever the item is used on something (blocks and maybe entities) then it's called, and the later is when the item is right clicked.
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