Is there an edible, colour-less, taste-less, odour-less, water-soluble, cheaply-producable gel, solid or other substance?
Are you talking about maltodextrin? It's a solid powder assembled from glucose units with no taste smell or color. It's commonly used as a carrier; it's used to dilute the actual sweet -flavored molecule in Splenda. 

If you're dealing with Hot oatmeal, you could add methyl cellulose to gel it up. Upon mixing with cold milk it will melt, acting like reverse jello; solid when hot and liquid when cold. 

You asked a rather strange question, I'd recommend instant oatmeal to prevent a possibility of bacterial/fungus growth when storing oatmeal for extended periods of time.