gulpfile.js

// Abaixo com a versão Gulp 4

var gulp = require('gulp');
var sass = require('gulp-sass');

gulp.task('sass', gulp.series( function() {
    return gulp.src(['node_modules/bootstrap/scss/*.scss', 'src/scss/*.scss'])
    .pipe(sass()) // converter o Sass em CSS
    .pipe(gulp.dest('src/css'));
}));

gulp.task('watch', gulp.series( function() {
    gulp.watch(['node_modules/bootstrap/scss/*.scss', 'src/scss/*.scss'], gulp.parallel( ['sass'] ));
}));

gulp.task('default', gulp.series( ['sass', 'watch'] ));


// Abaixo com a versão Gulp 3
/*
var gulp = require('gulp');
var sass = require('gulp-sass');
gulp.task('sass', function() {
    return gulp.src(['node_modules/bootstrap/scss/*.scss', 'src/scss/*.scss'])
    .pipe(sass()) // converter o Sass em CSS
    .pipe(gulp.dest('src/css'));
});
gulp.task('watch', function() {
    gulp.watch(['node_modules/bootstrap/scss/*.scss', 'src/scss/*.scss'], ['sass'] );
});
gulp.task('default', ['sass', 'watch'] );
*/