A payment gateway is the software that securely captures and tokenizes a card on your website or invoicing tool. Authorize.net charges $25/month plus 10¢ per transaction for the gateway alone — separate from any processing rates. NMI charges similarly. These were essential 10 years ago. Today, modern processors (Stripe, Helcim, Stax) include their own gateway free.
If you're paying $25+/month for a separate gateway, ask whether your processor offers a free native alternative. Often the answer is yes and the savings are immediate.