i have an albino rainbow shark that likes to hang out in an upside down vertical position behind one of the filter intakes every so often. this itself in your shark could be normal behaviour but the inflamed gill definitely needs some attention. is there anything in the tank he could have snagged his gill on? is it just one gill or are both affected?
i'd be doing extra large volume water changes, daily if you can, for at least a week to see if the extra clean water helps his gill to heal up. if it still looks red or inflamed or has gotten worse then you need to look into meds, most likely something in the anti-bacterial category.