考虑到你已经有了一个gulpfile.js，下面的代码应该可以工作，尽管还没有测试过。

const gulp = require('gulp');
const { spawn } = require('child_process');

// Task to build a project
function buildProject(projectPath) {
  return new Promise((resolve, reject) => {
    const buildProcess = spawn('dotnet', ['build', projectPath]);

    buildProcess.stdout.on('data', (data) => {
      console.log(data.toString());
    });

    buildProcess.stderr.on('data', (data) => {
      console.error(data.toString());
    });

    buildProcess.on('close', (code) => {
      if (code === 0) {
        resolve();
      } else {
        reject(new Error(`Build process exited with code ${code}`));
      }
    });
  });
}

// Task to build all projects under the Feature folder
gulp.task('build-feature-projects', async function () {
  try {
    await buildProject('./FeatureFolder/Project1');
    await buildProject('./FeatureFolder/Project2');
    await buildProject('./FeatureFolder/Project2/Project3');
    console.log('All feature projects built successfully.');
  } catch (error) {
    console.error('Error building feature projects:', error);
  }
});

// Default task
gulp.task('default', gulp.series('build-feature-projects'));