Could be a simple question but I am full of doubts right now about adding Google Analytics Tracking ID to GitHub page.
I am using G
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 )
index.html from listed filesHEAD 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 )