org.springframework.http.codec.multipart.Part.content() returns a Flux<DataBuffer>, meaning nothing happens until you subscribe to this Publisher.

If your code can be executed in a blocking way without causing errors, you can refactor it like this to get the String result:

public void parseFilePart(Part filePartMono) {
  List<String> parts = 
    filePartMono.content()
    .map(dataBuffer -> {
          byte[] bytes = new byte[dataBuffer.readableByteCount()];
          dataBuffer.read(bytes);
          DataBufferUtils.release(dataBuffer);
          return new String(bytes, StandardCharsets.UTF_8);
    })
    .collectList()
    .block();
  //do what you want here with the Strings you retrieved
}

If you're sure that the Flux<DataBuffer> will always emit 1 single DataBuffer, you can replace .collectList().block() with .blockFirst() and obtain a String result instead of List<String>.

If your code can't be executed in a blocking fashion, then you could refactor it like this:

public void parseFilePart(Part filePartMono) {
    filePartMono.content()
    .map(dataBuffer -> {
          byte[] bytes = new byte[dataBuffer.readableByteCount()];
          dataBuffer.read(bytes);
          DataBufferUtils.release(dataBuffer);
          return new String(bytes, StandardCharsets.UTF_8);
    })
    .subscribe(resultString -> {
      //do what you want with the result String here
    });
}

P.S. I didn't test your implementation to convert DataBuffer to String, so you might have to double-check that now that it's actually invoked

You could use the ModifyRequestBodyGatewayFilterFactory which I believe is included in Spring Cloud Gateway 2.0.2 which is part of Finchley.

For Ex:

@Override
public GatewayFilter apply(Config config) {
   return (exchange, chain) -> {
        ModifyRequestBodyGatewayFilterFactory.Config modifyRequestConfig = new ModifyRequestBodyGatewayFilterFactory.Config()
                .setContentType(ContentType.APPLICATION_JSON.getMimeType())
                .setRewriteFunction(String.class, String.class, (exchange1, originalRequestBody) -> {
                    String modifiedRequestBody = yourMethodToModifyRequestBody(originalRequestBody);
                    return Mono.just(modifiedRequestBody);
                });

        return new ModifyRequestBodyGatewayFilterFactory().apply(modifyRequestConfig).filter(exchange, chain);
    };
}