Has the fish always had a missing gill plate?
Are you familiar with the nitrification cycle? Article, link in my signature explains the cycle and many other helpful things. You might not need them but I recall after a long time of keeping fish I learned things too, I hadn't known about
If the fish didn't just grow up from a baby with the eroded gill plate, then it would look like it is a bacterial infection.
Flavobacterium branchiophilum is one, F. Columnaris, they are both gram negative so would need medication which treats gram negative infections.
Although recently I have read that often gill flukes cause the damage allowing infection to begin with.
And you would want to treat for that if you suspect it.
This article is fyi useful for the names of the bacteria and how it affects the fish. Talking about game and farm raised fish. But the disease aspect /effect is the same.