Is it a cage or an all glass/persex tank? Aquariums make better housing for gerbils as their ability to chew anything, means they can get their teeth round metal bars in a hamster cage and eventually either break their teeth or escape. Glass sides allow no purchase for their teeth and they can't climb up it. They can also figure out how to open hamster cage doors by pushing against the wire push lock.
Aquariums cater better as they are deeper and the bedding doesn't spill out at all. I use a 20 gallon tank. Filled half full with a mixture of shavings, playpit sand, potting compost and shredded straw or cardboard. The resulting mix holds different layers and as the gerbils tunnel into it, the burrows and tunnels they produce are self supporting because of the sand and compost and the straw and shavings keep them warm and snug.
This is the most natural habitat for a gerbil as it would be in its wild lifestyle, living in Mongolia and south america in mountainous desert regions. The habitrail and brightly coloured obscenities they sell as hamster or gerbil cages are not just cruel to the animal, but can cause phycological deformities and mental issues in the animal as they mature which shortens the lifespan. Its like sending a kid to Disneyland for the rest of its life and telling it everyday its living in the real world.
A tight fitting lid is essential as gerbils are great escape artists. As regards to toys, we call them toys because thats what we see them as, but a gerbil doesn't. It just sees them as another completely inanimate object in its environment. Instead of buying houses and other such stuff, use tree branches with lots of branches attached, but only use safe wood from a fruit tree which hasn't been sprayed. They can climb on them and will nibble the leaves if there are any. Cardboard tubes can be used but get thicker layered ones which will last longer and only use one per gerbil, don't clutter it up in there.
Food doesn't need to be fed in a bowl, because they will probably burry it anyway. Use a bowl if its easier but for a little enviromental enrichment, hide the food around the tank for them to seek out for themselves. This makes them work for their food and gives them something to do.
Monkey nuts hung on a string from the lid makes them jump and cling on while they free the nuts which makes there leg and back muscles work harder. In the wild they travel and climb great distances for small amounts of food and in captivity if they don't have something to make them work they get old and fat quicker.
If you use a water bottle it needs to be attached with the tube at head level, away from the bedding and in a way where they can't clamber up onto it. Don't use bowls for water as they can drown in it or just tip it over soaking the bedding. Gerbils have the ability to conserve their water intake during times of drought and so produce on average only two to three droplets of urine a day. This means they don't need fresh water on demand. If you feed each an inch sized cube or either carrot, apple or cucumber on a daily basis, it will provide it with all the liquid it needs for the day.
In the wild they go for days without water.
A nest box isn't needed neither is a house as if they have at least 8 inches of bedding to tunnel in, they will make there own nest burrow. Sometimes against the glass so you can see what they are up to underground. If you place a small handful of hay on the surface after they have dug down, they will collect mouthfulls of it to line the nest with.