We can use git subtree to do this. And also nest git commands to execute force push (Reference).
git push origin `git subtree split --prefix subfolder-name-here master`:gh-pages --force
Put this command to package.json file like bellow.
{
"name": "My App",
"dependencies": { ... },
"scripts": {
"deploy": "git push origin `git subtree split --prefix subfolder-name-here master`:gh-pages --force"
}
}
After that, we can use npm run deploy command to deploy subfolder to gh-pages branch.