A circuit-level gateway is a type of firewall technology that can be implemented across all layers of the OSI model, including the application, session, transport, network, and presentation layers. This type of firewall monitors TCP handshaking and session fulfillment between packets to ensure that the session is legitimate.
Circuit-level gateways are effective because they do not inspect the packet itself, but rather the transmission attributes to ensure a trusted session is established.
References: This information is based on the firewall technologies' capabilities as they relate to the OSI model layers, which is a part of the Certified Network Defender (CND) course material provided by EC-Council1.