For Spring Cloud Gateway MVC (without webFlux), an equivalent of AbstractGatewayFilterFactory (Gateway Reactive Server) is needed, which allows writing a custom filter and using it by specifying it in the YAML manifest, as shown in this StackOverflow question.

Currently, to use a custom filter, I am forced to write a @bean RouterFunction in Java.

Refer to the filter in Gateway Reactive Server here: Spring Cloud Gateway - Writing Custom GatewayFilter Factories.