A good gerbil cage should provide everything your little pets need. It needs to be of a suitable size and depth, made of a material the gerbils can't nibble their way out of. Don't underestimate their ability to nibble their way out of all thin plastics and wood!
A Gerbil's enclosure is where they'll spend most of their time - so make it a good one!
There are many different small pet cages on the market. They don't come better than Omlet's Qute Gerbil Cage, though. Check them out as your first port of call when sourcing a cage.