Excuse my speaking in English measurements - The GM 5561T, if what I found is correct, appears to be barely longer than 5 feet, and if I did my conversion correctly, you are about 6 feet? The head may add a couple of inches and the lens/camera itself would add several to get the viewfinder near eye level, but I would think it would be too close, especially if you are going to operate on unlevel surfaces where the monopod would rest a couple of inches lower than your feet. Or if you are going to be pointing the camera upward a lot, you'll want to raise the monopod rather than crouch down frequently or for extended periods. I'd go with the longer monopod.
But both of those are about four or five times more money than I've ever spent on a monopod. I use a good sturdy Manfrotto/Bogen, and it supports an older 400 2.8, about the same weight as the 800 5.6 you are talking about. (Personally, I don't 'get' the attraction to Gitzos and their twist tightening systems, especially if you are going to be adjusting the height often - and some of their monopods have not 3 but 4 (or 5, like the GM5561T) sections to deal with).