Some Thoughts and Options

Your question seems to leave it to interpretation. Too many open ended answers possible, and many great answers will be provided. However, based on what I think you are doing, here are some options:

• Use an external config file, and use globbing patterns for each, and exclude what you don't want.
• Create a gulp task that copies some source files and moves them to your dist/a, dist/b, etc.
• Turn your reusable code into a node_module and 'require' it in each apps src.
• Turn your reusable code into a git submodule and use it as a dependency for each app. Works well with the previous point.

A Gulp Task to move files and folders

For exactly what you asked, all sharing one repo (I do highly recommend separate repos though to simplify the tasks), here is a gulp task you can use to move files around. It's very simple.

This example / suggestion will have two parts for convenience. The task, and a config section to manage it. You can either use it in the gulpfile, or as an external file which may be easier to manage if you are using the same gulpfile for three projects.

You could also use merge-stream to do the same task to multiple src & dest:

npm install --save-dev merge-stream

gulpfile.js

// =========================================================
// Projects: Project A, Project B, Project C
// info: 
// 
// =========================================================
// ------------------------------------------------ Requires
var gulp = require('gulp'),
    merge = require('merge-stream');

// -------------------------------------------------- Config
var config = {
    // These are some examples
    reusable: [
        './src/reusableCode/scripts/**/*.js',
        './src/reusableCode/styles/**/*.css
    ],
    productA: {
        src: [  ],  // Add product A src
        opts: {  }, // Add product A options for gulp tasks,
        dest: './dist/A'
    },
    productB: {
        src: [  ],  // Add product B src
        opts: {  }, // Add product B options for gulp tasks,
        dest: './dist/B' 
    },
    productC: {
        src: [  ],  // Add product C src
        opts: {  }, // Add product C options for gulp tasks,
        dest: './dist/C'
    },
}

// --------------------------------------------------- Tasks
// ...

gulp.task('moveSrc', function(){

    var prodA = gulp.src( config.reusable )
        .pipe( gulp.dest( './dist/A' ) );

    var prodB = gulp.src( config.reusable )
        .pipe( gulp.dest( './dist/B' ) );

    var prodC = gulp.src( config.reusable )
        .pipe( gulp.dest( './dist/C' ) );

    return merge( prodA, prodB, prodC);
});


// A task to move folders and files without `merge-stream`
// gulp.task( 'moveSingleStream', function() {

    // gulp.src takes the src into a stream, and output the stream
    // just by using gulp.dest . This moves files and folders around
    gulp.src( config.reusable )
        .pipe( './dist' );
});

// --------------------------------------------------- Build
gulp.task( 'build', [ 'moveSrc', 'otherTask1', 'otherTask2' ] );

You could also skip the reasuable array in config in this example, and just add what you want to move in each products src array. Whatever works.

Now that being said, I would suggest separate repo's and using that reusable code as node modules, but you can use merge-stream to simplify the process of using the same tasks to do multiple things. I hope this helps.