Unless you have a very large container, it is never going to attain the very bold proportions it would get if planted in the ground. A local nursery has a mature Gunnera manicata in a big wooden box (approx. 4' square and perhaps somewhat deeper) and even it is smaller in scale and produces smaller foliage than similar ones do inground. And maintaining sufficient soil moisture in any sized container during the warmer months of the year may be a challenge.

Curious why you posted this question here.....you do know that Gunnera is not a lily??