Just want to throw it out there that this is still a problem in 2021. The walls are actually invisible as the game gives me a "wall does not exist" message when deleting them, but Sims on the lot will still act as though there are no walls (shooing other sims away when they shower, etc.). The easiest fix that you'll see right away is to use the wall tool to delete the whole wall with missing sections (ctrl+click+drag), then put the wall, paint, and doors back. I'm so sorry to anyone new to the game that doesn't even have a functional gym in their base game world.
This is still an issue. Why isn’t anyone talking about this? Seems like such an easy fix…. What is up with EA….
I just decided to give makeovers to these older lots and had the same issue with this lot. No matter how I try to redraw those walls, only the two towards the center actually stay.
I open the wall tool. Draw the wall over the missing walls and existing walls. I then grab the dropper tool and grab the orange paint. Cursor over one of the walls that is there and hold shift and click. When I first hold shift it highlights all the walls. When I let go some of the walls are still missing. Now go back to the world map view or whatever it’s called. And go back to the gym. And the walls will be there!
I rebuilt both base world gyms a while ago. Even after you've done fighting M&S with its broken walls, there's still not enough washrooms, and having the door at one end of the washroom area means NPC sims all queue up to use the closest room and then get upset that they can't. Rebuild with more washrooms and the door to the washroom area either in the middle or even better one door at each end, and the whole thing works so much better! Then save to your personal library and replace the original builds in every new save.
But yes. It's a key basegame building, one of the first public buildings most new players will encounter. It's been broken how many years? And they've never bothered to fix it.