interesting question....it should be almost gluten free..the only place a little might come from is the wine yeast listed in the ingredients. Beer yeast does contains small levels of gluten, but I would have thought wine yeast would contain none or close to none if the wine yeast has been grown up on grape juice/sugars (and not malt extract).
This from a google search which brings up marmite since its made for (beer) yeast extract:
A Unilever statement via Marmite's PR reads:
'Marmite hasn't changed at all over the years, we do not add gluten however gluten is in the product as it is a natural product and is contained within the ingredients [sic]. There are 60 ppm (parts per million) which means we can't say it doesn't contain gluten however it is a very low level [sic]. People who have been eating it for years and have had no problems don't need to change anything.'
If you are making wine and are worried, I'd expect the level of any potential gluten in the nutrient to be tiny in the finished wine, given marmite, which is concentrated yeast extract from beer, only contains 60ppm gluten. Wine is normally considered gluten free.
Not sure if this helps...may be worth shooting an e-mail to youngs.