We are happy to assist you with your requirement.
We can integrate any third party payment gateway to your Shopify store as a checkout option. (Even though they are not listed on Shopify gateway list)
Once we install your prefred gateway on your Shopify store. it will list under Alternative payment gateway list.
We only require your preferred payment gateway Developer API documentation and a test merchant access to complete the development & integration.
DM me or share your contact email ID for more details.