how do i care for my gunnera

This form of Gunnera is really a water margin plant - yours looks as if its drought ridden. It needs to be grown in moist soil, and should put on up to 6 feet of growth minimum by mid summer, so yours has suffered.
It will need protection in winter - usual way is to fold the leaves over the crown of the plant, but because yours has grown so poorly, that's not possible, the leaves are nowhere near big enough. I suggest you put a mulch of some kind (bark chips, whatever) over the top of the plant to a depth of 3 inches, and then cover that with fleece, anchored down. Uncover in the spring. You may need to move it to get it to grow properly ,or you need to keep it well supplied with water during the growing season.