Since our client had decided to use 'Gulp/SCSS' isnteadinstead of 'Grunt/Less' approach. I did some analysis and got this working. More testing is still ongoing, but I will list the changes I did on high level:
Seperation of modules: We received all UI files based on 'gulp/scss' from our design team. It was important to seperate the 'html-design' and 'Build-files' from the design. The build-files need not be updated frequently, so it contains the files like - node.exe, bower components and Node modules. Rest of the files and folders like assets, gulpfile.js, package.json etc were added to html-desin zip file.
Changes to output directories: I tried to re-use the exisiting tridion structure groups for compiled output of gulp task. So I modified the output paths of gulp tasks to match output structure of the grunt tasks. for eg: The output of scss-css build will be at '\dist\system\assets\css' instead of '\assets\css\build'. This way, there is no much need to modify the tridion structure. As an alternative, we can keep the gulp output paths as it is and create respective structure groups in Tridion.
Changes to TBB: The TBB unzips the design content and runs a command to process the gulp tasks on server. The command for Grunt had arguments like:
Arguments = "/c npm start --color=false"
[Still couldn't figure out the significance of '--color=false' :) ]
For gulp, I had to change the working directory and also replace arguments in the command with:
Arguments = "/c gulp.cmd"
Other than this, there were some minor changes like using _custom.scss instead of _custom.less etc
With these above changes, I was able to successfully publish the HTML design configuration containing the gulp/scss build.