"Gas bladders" and lungs are similar organs, although they serve different functions.
Both of them develop from a location in the upper throat i.e. the pharynx and are used to manipulate gases. They both contain a good blood supply.
However, the lung is used for breathing and is often found in land vertebrates, while on the other hand, the gas or swim bladder is used in the regulation of buoyancy and the latter is found in all other bony fishes e.g. perch.
Perch use the swim bladder to control their buoyancy by filling it with oxygen from their gills using a gland, which is known as a gas gland. This expands the bladder, thereby pushing the fish toward the surface of the water.
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