input: [It might be...] a stroke. Gerbils do seem to be prone to this. Sometimes, gerbils will retire to their nest after a stroke for a day or two, then emerge seemingly better. Sometimes the damage is obvious and permanent. In either case, s/he is likely to have another stroke later on. The key now is quality of life. As long as your gerbil can continue to enjoy normal, gerbilly things, like digging, grooming, eating and chewing, then just enjoy your time together. However, over time you may see his or her abilities deteriorate. Dont wait until your gerbil is curled up unmoving and you are trying to hand-feed it seeds while it bites you. Once your gerbil cannot enjoy life, see the vet for humane euthanasia. [My Gerbil...] 
output: seems to be paralyzed on one side; is dragging around, or can't fully open one eye