Upgraded to Craft 3 and lost my Commerce Gateways
Commerce 2 moved all payment gateways to plugins.  You can see the relevant bit in the docs here: https://docs.craftcms.com/commerce/v2/payment-gateways.html

There are currently seven 1st party plugins that add payment gateways and the two most popular, Stripe and PayPal are free.  You can see the table for the remaining in the above link.  All of these are currently in the Plugin Store. 3rd parties can additionally write plugins that work with other payment gateways.