Nick here from Shopify.
Shopify doesn't list which payment gateways are direct or external. The reason for this is because available providers change frequently, sometimes leading to out-of-date information which could be misleading. Because of this, the best way to find out if a gateway is direct or external with Shopify is to contact the gateway's themselves as it is their gateway and they would be the ones who control this aspect.
In terms of creating your own payment gateway, you can look into the steps for how to do this here as well as completing the Shopify SDK payment gateway form here. Something to note is that I'm not 100% sure if it's possible to build an external gateway and that access to the Hosted Payment SDK is limited. It might be worth filling out the form and then asking about building an external portal from there if you get accepted.
I know this might not be an exact answer to your question but hopefully, it points you in the right direction.
All the best, Nick
Let's say I want to build a small private app which accesses the checkout page and adds an option to the manual payment options(by code) and when the customer selects this option, he will be redirected to a certain URL that I specify. How could be it done?
PS: nevermind considering a payment gateway, it is for a different purpose like : Offering *Pay Later* or *Slice your Payment*
Ah ok, in this case, @Anzimaty, your best bet would be to look into making a Shopify app that offers a partial payment feature.
I am personally not the right person to ask about app development but there are a couple of helpful guides which you might find useful. One is about Making Shopify apps and the other is about Building a Shopify app with Ruby and Sinatra.