The drywall is behind baseboard cut the drywall out and replace reinstall baseboard. If u cut out below baseboard height u dont need to tape sheetrock They sale 2x2 size sheetrock for repair. Measure your thickness probably 1/2 inch. You want get smell out of sheetrock that has been saturated. Its a paper product when paper gets wet the harder section crumbles. If u need to cut higher above baseboard height then u would need to tape / mud. YouTube videos

The only real solution is to cut the drywall and patch.

Since replacing drywall seems to be your only option, I'd try one more thing first.... I recall reading/seeing where someone couldn't get the cat urine smell out of their rug and they were told that only boiling water will kill whatever enzyme/thing causes the horrible smell. I wonder if it would effectively kill the smell if you hit the area with high heat or even steam.... or boiling water. But if I tried it I'd be ready to clean it up quickly!

When using the enzyme solutions, I find that you really have to soak the item with the solution to get it to work. Lightly spraying isn't going to do it.

Have you tried Odoban? Their instructions specifically call for NOT saturating, but lightly misting and letting dry and repeating if needed… Might be worth a try although I know cat urine is almost impossible to get rid of.

Try Zinnser's BIN (pigmented shellac) primer. The one with the red label.

I would use two coats of BIN.

If you're looking at last resorts: two coats of oil-based primer.

I would first invest in a black light to see exactly where the urine is and how large an area you're really dealing with.

The area that is painted may not be that saturated with urine but the area of unsealed/unpainted mud and drywall (below the paint line) is probably where the worst odor is. If that's the case, then trim away the mud and unpainted drywall, treat with enzymes made for cat urine, let it dry thoroughly (put a fan on it) and then coat with oil-based primer.

You may be able to remove a lot of the mud with a wet rag.

To replace the baseboard you'll need to do as Rehabit described and fill in some spots with pieces of drywall.

Is that raised, open area on the corner damage from urine?