What does Timestamping of GPIO pin means and how to achieve it?
There is some confusion here. The link that you gave is not very clear what they mean.

Do you know what a timestamp is ? It is a stamp that can be set to a certain date, sometimes added with the time. With some ink, the timestamp can be set on important documents. It was used quite often 30 years ago.
Some timestamps had a clock inside to keep it automatically at the right time.

The general meaning in program code is to remember the date and time for a certain event. For example a pin changed from low to high at october 6, 2017 at 22 hours, 0 minutes, 10 seconds, 123 milliseconds. 
The timestamp does not have to be the real date. The value of a (hardware) timer can be used as well.

With Arduino code, the millis function can be used for a timestamp.

When dealing with complex code, for example a full linux operating system, it can be useful to know the time that a certain event happened. It is important to know how accurate the timestamp is. Perhaps the "5 µs" in the link that you gave is the accuracy.