Key Differences:

Network Type: GSM Gateway uses mobile networks, while VoIP Gateway operates over IP networks.
Primary Use: GSM Gateway is for mobile communication, whereas VoIP Gateway facilitates integration between IP telephony and legacy systems.
Cost Efficiency: GSM Gateways are used to reduce mobile call costs, while VoIP Gateways reduce overall communication costs by leveraging internet-based telephony.
In summary, while both gateways aim to optimize communication and reduce costs, their applications and underlying technologies cater to different aspects of telecommunication needs.