I've read that:
Gateways are basically protocol converters, facilitating compatibility between two protocols and operating on any layer of the open systems interconnection (OSI) model.
I don't understand what protocols gateway converts. Why is it important to convert protocols? Isn't TCP/IP protocol stack common for all devices that take part in network transmission of data?
"Gateway" is a broad term which is used in different ways in different contexts. "Protocol" is a similar broad term with context specific meanings.
What you cite would for example apply to gateways translating between IPv4 and IPv6 (different IP protocols), between SIP, POTS or H.323 (different telephony protocols), between HTTP/2 and HTTP/1 (different versions of HTTP protocol), WebSockets and plain TCP sockets (different protocols for bidirectional communication) etc.
Note that other common uses of the term Gateway in the context of network don't involve converting protocols. For example Secure Web Gateway or Secure Mail Gateway keep the application protocol but inspect the protocol payload. For even more meanings see Wikipedia: Gateway.