input: [It might be...] if you observe this behavior when your gerbil is standing on its hind legs, and if your gerbil is pink-eyed and sways slightly when s/he does this, it is probably okay. Pink-eyed gerbils sometimes display this head-tilt & sway behavior: some say it is a way of them focusing their poorer eyesight. If however this behavior comes on suddenly, is true at all times (not just when standing up prarie-dog style), or seems to also affect the gerbils behavior or energy level, or is accompanied by kicking or scratching at an ear, your gerbil could have an ear infection or a middle-ear tumor. See the vet. [My Gerbil...] 
output: seems to be tilting its head to one side