Going to contact them next week and see. Might be a decent move to put a little money in there if I can take loans against it (in a cheap way), a couple years down the line.
It doesn't look a great idea to me: do you really want to buy bitcoin with a 20% premium only to have a cheaper loan?
I am not sure about your fiscal and regulatory framework, but I guess the saving you can have on the loan side of the deal is minuscole compared to the premium you are going to pay on the "grayscale secondary buying" leg.
A completely different consideration would have been made if you were an accredited investors with some restricted GBTC shares. There could be a good deal.
As I said in the OP, there are a large numbers of subject that cannot hold BTC on the first place. For them the only way to get exposure to BTC is getting trough Grayscale or similar products.