When you are editing a product and click on the Payment Gateway screen, you’ll see options where you can choose which payment gateways are available for this product.
You can also set the product to live or test mode. In the above screenshot you can see that it is in Test mode and that I have enabled the Stripe payment gateway. This screen will only show the Payment Gateways that you have active on your site – in this site, if I had PayPal integrated, it would have showed as an additional option just like Stripe is showing.
Here is an example from another site where I have Stripe and PayPal integrated:
If you are using multiple products in a funnel, you’ll want to ensure that your upsell and downsell products have the same options enabled.