Welcome to GoY, Wannabe!
What part of the U.S. are you gardening in? If your summer nights are too warm, Gunneras will rot no matter what you do. Also, it is a semi-aquatic plant, and needs ample water, especially in containers. If allowed to dry out too much, it will rot when water is restored, because of root damage.

Given the size that gunnera manicata grows to I can't personally see how you expect to grow in a pot. Its natural environment is growing beside, and often partially in, running water.