1) is using a Payment Gateway anyway compulsory to make online payment works as our consultant informed us? 2) which Payment Gateway your idea will work with?

1. Using a payment gateway is not compulsory, unless you want to process payments automatically. In this latter case, indeed it is compulsory, otherwise Octorate would not be able to collect payments.
   If you do not set up any payment gateway, the only thing you can do in Octo is viewing customers' Credit Cards.
2. Here is the list of platforms integrated with us:

Thank you so much and have a great weekend!