Great question — answering as the UphillTech team:
I think the answer to your question is another question. If you coded up a working Payment Gateway not currently supported by Shopify, using Active Merchant (which is currently supported), would Shopify accept it, and thereafter would merchants be able to choose it? That is a tough one. You can try! You might succeed even. All those working Gateways in Shopify did not get there exclusively from Shopify.
Full answer by David Lazar: https://stackoverflow.stackexchange.com/a/34938128 (CC BY-SA 3.0).
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