A minimal example of using marked with gulp.js without the need for a separate package to do it for you.

gulpfile.js

/* 
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*/

/**
 * This is a minimal example which doesn't use an external package
 * to convert markdown code to HTML using marked, with gulp.
 * 
 * I wrote this as:
 * a) there is very little documentation on how to do so.
 * b) downloading a package to do this is ridiculous given that it just calls one function.
 * c) pure frustration.
 * 
 * The meat & potatoes are in markedToHTML. Hope it helps.
 */

/**
 * This is a minimal example which doesn't use an external package
 * to convert markdown code to HTML using marked, with gulp.
 * 
 * I wrote this as:
 * a) there is very little documentation on how to do so.
 * b) downloading a package to do this is ridiculous given that it just calls one function.
 * c) pure frustration.
 * 
 * The meat & potatoes are in markedToHTML. Hope it helps.
 */

const { watch, src, dest } = require("gulp");
const { marked } = require("marked");
const Path = require("path");

var Transform = require("stream").Transform;

function markedToHTML(options)
{
    //See <https://github.com/gulpjs/gulp/blob/master/docs/writing-a-plugin/README.md> for
    //more guidance on what is going on here.
    var transformStream = new Transform({ objectMode: true });

    transformStream._transform = function(file, encoding, callback)
    {
        var error = null;
        var output = file;

        //Get the contents decoded to a string we can use
        const content = file.contents.toString(encoding);

        //Parse it from MD to HTML
        const html = marked.parse(content, options); 

        //Rename from .md to .html.. without gulp-rename
        let filename = Path.basename(file.path, ".md");
        file.path = Path.join(Path.dirname(file.path), `${filename}.html`);

        //Set contents: output must be a File, not a string
        output.contents = Buffer.from(html, encoding);
        callback(error, output);
    }

    return transformStream;
}

function compileMD(cb)
{
    src("*.md")
        .pipe(markedToHTML())
        .pipe(dest("dist/"));
}

exports.default = function() 
{
    watch("*.md", compileMD);
}