Well, I'm not exactly sure what you're after here, but let me see if I can guide you through this. Gills are what help fish breathe and get the oxygen they need to conduct cellular respiration. The oxygen is dissolved in the water they are in, and as the water passes over their gills, the oxygen passes through the thin gill membrane into the fishs blood. At the same time, carbon dioxide, a waste product of cellular respiration, is released from the gills into the water. That would be your gaseous exchange. You mentioned one of the gills was immersed in water, while the other was not. Something important to remember is the gill only works at removing oxygen from water, it is not set up to remove oxygen from air, the way our lungs do. So my guess is the gill in the water is a bright red color, indicating the presence of oxygen, while the gill in the other petri dish without water is a dark, dull red, indicating oxygen-poor blood, high in carbon dioxide. That would be a logical conclusion to reach, based on the color observation difference between the two gills.