[ERROR] Gulpfile not found: ./gulpfile.js

I encountered into the same problem, and after many unsuccessful tries, I added one line to node_modules/@ionic/cli-plugin-gulp/index.js:

if (e.code !== 'MODULE_NOT_FOUND') {
     throw e;
}

console.dir(e); // ADD THIS

throw new cli_utils_1.FatalException(`Gulpfile not found: ${chalk.bold(cli_utils_1.prettyPath(gulpFilePath))}\n` +
                `You can set the ${chalk.bold('gulpFile')} attribute in ${chalk.bold(cli_utils_1.PROJECT_FILE)} for custom Gulpfile locations, otherwise the default Ionic Gulpfile can be downloaded from ${chalk.bold('https://github.com/ionic-team/ionic-app-base/blob/master/gulpfile.js')}\n\n` +
                `Or, if you no longer use gulp, you can remove the CLI Gulp Plugin:\n\n` +
                `    ${chalk.green(gulpPluginUninstallArgs.join(' '))}`);

then run ionic serve again, the console can print out why it fails, we could try to resolve them now.

In my case, it's because of some missing gulp related packages (gulp-bower gulp-minify-css gulp-rename gulp-concat)

We should consider more accuarate error output instead of misleading Gulpfile not found for every error.