I have been thinking about getting two gerbils. I have a 30-gallon aquarium that I was planning on using, but is 30 gallons too big? I would fill it with bedding for burrowing and plenty of toys and enrichment.
I would recommend that you get two gerbils from an American Gerbil Society breeder that have been bred for “gerbil gentleness.” This means they are bred for many generations to be easy to introduce, to not declan and to have docile natures. Let the breeder know you intend to keep them in a large setup.
Getting a pair and not a trio is a good idea because two gerbils tend to get along better than three. Some gerbils can get territorial if the setup is partitioned into sections or separate territories. If you create a housing arrangement where there are clearly playing spaces (i.e., that are out in the open) and only one hiding/sleeping spot, I think your gerbils will be thrilled with the space and stay well-bonded.
There are many YouTube videos of gerbilariums you might want to look at for ideas. Place their one permanent nest box in one corner and then for the main tank give deep bedding to dig in, timothy hay to nibble on, a dust bath to roll in and a paper towel tube to crawl through and gnaw to pieces. A climbing branch from the reptile section of a pet store is a huge hit as well.