Update: Added steps descriptions for others

Solved it:
had to include username.github.io (link that I want to track) in Google Analytics website section.

you can check GitHub help page here

After that I was provided with an Tracker ID.

Note: You can easily change or add more websites on Google Analytics page from your Google Analytics admin panel.

Update 2: - Adding Google Analytics Tracking ID to Already created Github pages (As requested by @avi-aryan )

1. Browse to your github pages branch - which would be something like -
   ( https://github.com/YourUserName/YourRepository/tree/gh-pages )
   
2. Then edit index.html from listed files
3. Now in within HEAD tag of index.html - paste your Google Analytics Tracking ID Script ( if have already signed up for Google analytics then you can browse it under admin and then tracking info tab )