included gcd.cpp

[arpitsinghania8]

Submission Checklist

• [ ] Your pull request targets the master branch of the repository.
• [ ] You have only one commit (if not, squash them into one commit).
• [ ] You have read the Contributing guidelines and your changes follow them.

Type of Change

• [ ] Bug fix
• [ ] New implementation

PR Description

Issue #[Add issue number here.]