When you win against a gerbil, it's like scoring a jackpot in the lottery. It's not necessarily that gerbils are smarter, it's just that they're extremely persistent. They constantly find ways to outmaneuver whatever setup you have, especially if its in resistance to something unhealthy or annoying that they do.
Example 1: My gerbil, Odin, has an annoying habit of "digging" against the side of his tank obsessively. He's not trying to get out, it's just in his nature to dig. I just don't understand why he's digging sideways and always in the same place. If you happen to notice that opaqueness on the bottom of his cage, congratulations - that was my attempt at stopping his weird habit. This is double-sided sticky tape. Now, if you also notice, it doesn't go all the way to the edge of the tank. This is because gerbils are persistent bastards. Eventually the tape would become unsticky, and once it did, he dug it right off the glass.
Example 2: This one was actually an unhealthy habit that I needed to stop. Odin had an obsession with chewing on those black metal bars. Eventually, the coating would come off (which I'm sure is toxic), and/or he would either break one of his teeth or get mouth sores, I was sure of it. Plus, it was extremely annoying. He loved to chew on the roof bars which I found very strange. Searching internet forums was not helping. As you can see, Odin already has plenty of things to chew and occupy his time, but he was choosing the bars instead. This time, I couldn't half-ass my approach. I'd already tried tape which only stopped him for a day. Plus, I didn't want to cover his whole upper area with tape.
Forum people, you need to listen to me right now. This works, this fucking works, and I'm going to share my secret with you. It's this: hardware cloth (they call it cloth but it's more like chicken wire). As I watched Odin chew the bars to death, I thought; look at how far his nose protrudes past the bars in order for his teeth to get on them. Gerbils have extreme overbites. If he can't get his nose over it, he can't chew it. And that was my revelation. Cover the bars with metal where the holes are too small for him to fit his nose through.
For the roof, I did the outside. I bought some Gorilla epoxy and used zipties to keep the two metals in contact because, while the wire mesh is bendable, it's too stiff to stay. It was a horrible experience that I choose not to repeat. I got the epoxy everywhere. It made drips down the bars and dried in droplets on the underside. However, that might be a partial reason why he's now leaving the roof alone. Oh, he tried, and lasted about 3 seconds. He tried again, lasted another 3 seconds. I was winning the battle. But, I knew it wasn't over. He also sometimes chewed on the sidebars, so I began to cover those as well.
[Side note: this project took about 3 days, so while I was doing this, I had the cage topper on my table and Odin only has his tank. I had to tape the water bottle to the side of the tank because I had no other way to attach it. When I went to bed on day 1 of the project, I got up the next morning and to my extreme rage, Odin had chewed through the tape, and then chewed a hole through his only water supply. I literally screamed at him. Picked him out out of his cage and cussed up a storm. This is why I don't have children.]
For the side pieces, I wanted to make it easier on myself and decided to cut them to fit the inside of the cage and just use zipties to attach them. I figured he might not be able to get his teeth on them if they were "skintight" against the bars. So far so good. He might just be biding his time though, who knows. Ever since I attached the sides, he has not tried to chew on the bars at all, even though he still has access to them from the front and back of the cage.
It may be too early to tell but, I think... I think I've won. I'm not quite sure what to do with myself. Every single battle that I've fought, I lost miserably (take the food dish for example. When it was in the tank, he'd cover it in his bedding. So, I put the food dish on top of his hut. As you can see, it is clear of bedding. I woke up the next morning with bedding in his dish. Gerbils are the most bizarre creatures.) but I really hope this sticks. I highly recommend anyone with this problem to try this out. You might finally outsmart your gerbil.