One of my fish (gold barb. . .adult, 2.5 inches long) has 1 gill that is very red / missing too. It looks like it would be very painful to the fish, but the fish is very active and eats fine too. It looks like a gaping open sore, bright red (like blood too), and you can see into where the gill used to be. I feel very bad for the fish, as the fish has actually been like this for a whole year now. It looks like it might have got bitten off by one of the other gold barbs in the tank (I just have a 10 gallon freshwater tank with 3 gold barbs only in it), or perhaps the gill got eaten away by bacteria.
I do a 50% water change once a month, and I put in aquarium salt 2 weeks ago (1 heaping tablespoon per every 5 gallons like the aq. salt directions say) but have noticed no change positive or negative.
I thought it could be "bacterial gill disease" or "open red sores" which can be cured with the antibiotic Erythromycin, but I tried putting in Erythromycin according to the directions, and that did not work. My other fish had "mouth rot" though, and the Erythromycin definitely did cure that.
I think it definitely wouldn't hurt to try Erythromycin (Petsmart sells it for $16, Petco does not sell it), just in case the red gill is caused by bacteria eating it away, but if that doesn't work, maybe the gill was just bitten off by another fish, and it can never grow back. If you or anyone else finds this helpful or not helpful or has any other thoughts on this, please let me know. Thanks!