There are not really pest issues associated with Gunnera.
If you have snails or slugs; this could cause this damage. Since they feed in the night they are rarely seen.
You could set out bait to see if the issue disappears.
I would also suggest treating the plant with Neem Oil. If there are pests on the plant, this treatment should take care of most insects.
Make sure you treat the top and bottoms of the leaves.