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
For anyone interested, if you are using Jekyll with GitHub pages, I just wrote a post showing how to correctly add Google Analytics Tracking ID to Jekyll.
You will find your Universal Analytics tracking code under Admin > Property > Tracking Info > Tracking Code.
Create a new file called analytics.html in the _includes folder found in your Jekyll website’s directory.
Add Google Analytics Tracking ID code to analytics.html.
Finally, open _layouts/head.html, and add {% include analytics.html %} just before the end </head> tag. Google recommends this placement to track all of the pages on your website correctly.
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 )