You may be giving your gunnera too much water, and perhaps too much sun. Mine are huge, but they only get watered a minute or two at a time once or twice a day (they grow in a mixture of peat and compost above a clay soil.)

How deep is the crown of the plant? If you have covered up the sides of the crown with compost,etc, you may have a rot problem. In Ireland, where there are some really MAMMOTH Gunnera, they look like they grew best at a streamside..with the crown well out of water, and the roots seeking the water. It is hard to overwater a Gunnera! You may have salt damage, if you have fertilized the plant heavily, and yet not poured the water THRU the root system. I used to pile a huge wheelbarrow full of straight Bunny manure around each plant, and then, water, water, water...they just burped and grewwwwwwwwww. Your Gunnera wants full sun, but will tolerate a small amount of shading. The leaf stalks will flop/bend in too much shade.

Is anyone in the Tampa Fl area, growing a Gunnera Manicata successfully? 6 months ago I bought my 2nd plant.The grower said don't plant in a planter. I did, filtered sun. It was growing but only 12" leaves. 2 weeks ago, I transplanted it into the ground with morning sun in April, lots of Miricle Grow potting soil and 1/2 compost. Leaves are dying from the tips inward. Not enough water? What?
Thanks for any help.

Crispy leaf tips usally means too much sun/not enough water.
Here is a link to a question under the "UK garden web" about requirements, etc.

Here is a link that might be useful: Why they grow so big there!