Can I use PayPal or another payment gateway instead of Stripe to bill my clients?

It depends on what you bill for.

SaaS plans: plans created in the SaaS Configurator (the plans that create a sub-account) and white label billing use Stripe and only Stripe, because they rely on Stripe features the other gateways do not have. As a white label agency, you connect your own Stripe account so your customers pay you for their plans.

Everything else: for invoices and other products, physical or digital, you and your clients can use other payment gateways, managed under Payments in the app. Connect them under Settings, then Integrations, then Other Integrations, then Payments. Options include Stripe, PayPal, Checkout.com, Braintree and Authorize.net.