Why gerbils measure space in depth
Gerbils come from arid country where they live in burrow systems, and digging is not a hobby for them but the central activity of their lives. A gerbil pair measures its home largely by how much workable ground it has: substrate deep enough to excavate tunnels that hold their shape, chambers to sleep and store food in, and enough floor area that two gerbils can establish their network without every tunnel collapsing into the next. This is why gerbil housing advice talks about depth of bedding before anything else. A pair with deep bedding will spend their days engineering; a pair without it will spend their days looking for somewhere to dig, and that restlessness drives most of the problem behaviour owners report.
The tank footprint and how the space is used
A proper gerbilarium is a glass or similar solid-sided tank, sized for a pair with a floorplan in the region of a large aquarium rather than a carry cage, because the whole surface becomes digging territory. Fill it so the gerbils get a deep bed of substrate, around 25 to 30 centimetres, built from layered bedding and hay that holds a tunnel when packed. Keep the layout low and stable: a water bottle fixed at the surface, a heavy food bowl on a flat stone or platform so it is not buried within the hour, and wooden shelters partially sunk into the bedding as starter chambers. A solid exercise wheel can sit on a raised section at one end. Tall plastic towers and tube systems add little, because gerbils would rather be under the ground you give them than climbing through accessories above it.
Where a standard small cage falls short
The small barred cages sold for hamsters and gerbils fail on nearly every point of that design. Their trays are shallow, often holding only a few centimetres of bedding, so a gerbil can dig for seconds before hitting plastic. What bedding there is gets kicked out through the bars onto your floor daily. Worse, gerbils in shallow cages frequently take to chewing the bars themselves, a stereotyped behaviour that can wear teeth and damage noses, and corner-digging against the tray for hours at a time. The cage may actually advertise more height and levels than a tank, but levels cannot substitute for the digging depth gerbils are built to need. Owners often buy the small cage first, watch the frustration behaviours, and then buy the tank anyway, paying twice.
Sizing up honestly before you buy
Plan the tank first and buy once. Clear a surface that takes a genuinely large footprint, in a spot away from radiators and direct sun, since glass tanks hold warmth. Budget floor space for the tank plus room to lift the lid and work, because full bedding changes are a two-handed job. If the only space you have fits a standard small cage, the honest conclusion is that the space does not fit gerbils, and a different pet will be happier there. Gerbils kept as a pair in a deep tank are engrossing to watch, precisely because the setup gives them a world to build rather than a box to pace. Sketching the layout before buying glass, bedding and a wheel prevents costly resizing later. For anything still unclear, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.