Will I need in app payments or can I use custom payment gateway?

I am developing app for sharing rides like BlaBlaCar. I have credit system that requires users to buy credits to actually use app. I am unsure if Apple will allow me to integrate custom payment gateway or I will need to integrate in app payments.. I wouldn't like to lose 20-30 % of money in in app payments from Apple. Are those credits considered as digital product or not? I am also planning monthly subscription so users don't need to "pay" credits for using app but have unlimited access to app for that month. Will that be a problem?