Where is your gunnera positioned? frankly would think it unlikely to be able to move a 15 year old plant!

You can though divide it any time now, in spring before the summer, so your best option might be to slice a section off that contains new growth, replant where you wish it to be with suitable compost etc then dig up and ditch the rest once new section has established. Also, light may have something to do with why it is growing forward and not where you want it to, so moving its position back may just cause the same problem in the future?? Is there any way you could improve the light levels by undercutting/lifting branches on other stuff?

or putting mirrors round that part of the garden . you can get reflective sheeting on a roll . every bud on your gunnera is a potential new plant . id move a couple just to make sure and maybe pot the others up to sell or swap .