How do I repair or replace a fascia gutter end caps?
That doesn't look like a "standard" gutter shape anyways. I doubt you'd be able to buy something to fit natively anyways.

What you can try is buying some sheet metal and cutting it to size with tin snips, making fold points on the sides so you can secure it with stainless steel screws. Then you caulk it up good. It should buy you some more time until you replace the entire gutter system.