I'll assume you've gone for G. manicata, the largest of the species, though G. chilensis(tinctoria, scabra) is good for those not quite so ambitious(about 6' dome).
I can't say for sure about the depth of root but that very much depends on the size of the plant, I've never tried to move or remove one, in any case, the books say not to disturb once planted and that suggests an extensive fleshy root system to me. It enjoys a good rich deep soil and wants to be in a semi sheltered place, although it does enjoy the sun ie keep it out of the wind to prevent the leaves getting damaged and torn. It must have a good continuous supply of water to the roots. It is a plant that will grow well in a bog, in sump conditions where everything else dies out. It doesn't need feeding but will enjoy a dosage of decayed cow manure every spring(nitrogen).

There is a plant 15 miles from here that reaches 12'-15' high by 30' across every year, but that's exceptional. Most of the others I know seem to be about 9' high. Cut the small flowers out as they form and this will give you bigger leaves. The flowers are not up to much anyway.

You should be OK in zone 8 but DO PROTECT THE CROWNS in winter. The way this is normally done is to cut the leaves off and invert them over the crowns, and tie them in place. I would be inclined to supplement with bracken or straw in the first few years until the plant gets established.

I would think you'll be 8-10 years before the plant reaches a dome of about 8' from a 1 gallon pot just now, but that very much depends on getting it established, happy and fed, and it not getting leaf damage in it's early years. You could be there in 4-5. I'd be interested to find out.

Cheers Greenmanpants

Thanks for all your great advise!!! I have printed it out for future reference!

Amalia

Greenmanpants... I also have a question...I live in z8 and can't seem to keep one of these alive. I created a bog area and they seem to turn to mush. am I keeping them too wet? The leaves just turn brown and shrivel up and die..I've already lost two and #3 isn't much behind.Thanks for any advice you can give. Dottie

I'm curious, when you say they turn to mush, do you mean after the winter??? They do need protection from frost, zone 8 is easily cold enough to kill them if you haven't wrapped them up warm for the winter, people here use just the leaves(zone9), but you can also use bracken and straw as insulation then wrap them in their leaves and tie them in place. Unwrap them when frosts are past and they'll look pretty dead but will spring into life again usually during May.

I would think young plants are much more susceptible to the frost than a big old 4 foot dome of crowns.

They don't grow actually in water but have access to ample water at the root, are you planting them a little too deep perhaps?? There's quite a few things grow ON a bog rather than IN a bog when you actually look closely at how they lift themselves out of the water, Rheum, Darmeria, Rogersia and Osmunda are all classic waterside plants with their roots having constant access to water but with their crowns actually above the water line.

Cheers Greenmanplants

Hi Greenmanplants...I lost my first one that had about a 5 inch crown for what ever reason I don't know...then I just got two more on a trade, they were smaller and looked great when they came, but now one is dead and the other's leaves have turned brown and shriveled up... so it's not from being cold as it is warm here in Chico. Maybe I am planting them too deep. Will try to lift it up and see if that helps... I really really want to have success with a gunnera. Thanks for your suggestions.Dottie