We want to conclude with the Acquirer from the USA. There will be no clients who are U.S. residents. Does we as an MSO (Hong Kong) PSP/Payment Gateway need to obtain GIIN?
Everything depends on whether the acquirer has some presence/representation in Hong Kong. If it does not, you might even be required to have a US tax ID or a GIIN as you say. Technically, foreign financial institutions and their branches are assigned GIIN s. However, again, the best way is to approach the representatives of the specific acquirer you have in mind.