I really cannot tell from the pictures, it looks to be a plastic case. I am sure it works fine and the thermometer probably goes up and down as the ambient gets warmer or cooler. Accurate, probably close enough.
Do yourself a favor and purchase a made in Italy B&G (brass and glass) spg, either a standard 2.5 inch or consider one of the 1.5 or 2 inches spgs if weight is a concern. Dive Gear Express or any of a number of other retailers or online stores should have them under different brands (Hog, Deep Six, DGX etc) at cheap enough prices for something that could last you many years and is much more rugged than a plastic unit.