In fact, I think most banks wouldn’t be happy even if they knew it was a gift, and might make the parents co-sign the mortgage. The idea is that the banks see the downpayment not just as a way for the borrower to have some real stake in the property right away, but also as proof that the borrower is capable of living within their means, with enough discipline to save over a decent length of time (and therefore should have the financial discipline to keep up on their mortgage and home maintenance, etc.). Contrariwise, if the borrower needs a gift in order to get a downpayment, that’s an indication that the borrower doesn’t have the financial discipline to save, and might be a bad risk.
So in your case, I don’t think the bank would be happy making a loan if they knew the situation. Look at it this way: if the bank is happy with you borrowing the downpayment from someone else, why wouldn’t the bank be even happier with you borrowing the downpayment from the bank (so the bank makes more of the profit)? Seems unlikely to get a mortgage without misleading (i.e. defrauding) the bank.
But I don’t know anything about commercial RE lending, so maybe it’s possible. In any event, I wouldn’t even consider this until I had a good lawyer look over all contracts and agreements. There are a lot of possibilities, and you needs someone to consider them all.