Elle here from Shopify.
Currently, this is not a feature that we offer through our platform. If you're looking to have certain products on a certain gateway for subscriptions, then there are apps available for this such as Bold Subscriptions.
May I ask why you are trying to accomplish this? If I understand your intent further then I can either provide more workarounds, or hand your feedback along to our developers for consideration.
Looking forward to hearing back from you!
I have this same issue. I have done a workaround for wholesale customers, it's working so far. But I need to add a monthly charge for one of the items we sell. Neither of these are offered on our storefront. So Bold Subscriptions doesn't work. I was thinking of offering a specific payment form (net 30 etc) for this ONE monthly charge item and keying the order myself thru Bold but that's not an option to specify by product.