gulpfile.babel.js

import gulp from 'gulp';
import gulpJsonTransform from 'gulp-json-transform';
import countryList from './config/countries';  // it's an array of string

// omit clean, distribute etc

function patchManifest(done) {
  const tasks = countryList.map((countryName) => {
    // Right here, we return a function per country
    return () =>
      gulp.src(`./build/${countryName}/manifest.json`)
          .pipe(gulpJsonTransform((data) => {
            data.homepage_url = .....; // Something different here by country
            return data;
          }))
          .pipe(gulp.dest(`./build/${countryName}/`));
  });

  return gulp.series(...tasks, (seriesDone) => {
    seriesDone();
    done();
  })(); 
}

const demo = gulp.series(clean, distribute, patchManifest, archive);
gulp.task('demo', demo);