Learnings from moving from MBR to GPT, multi-booting and random stuff

This weekend, I converted all my 3 disks (2 HDD and 1 SSD) from a MBR based partition table to the newer one GPT. This was some work as I had to ensure that all my existing OSes ( Debian 8, Kali and Windows 10 ) run smoothlessly.

I am sharing some of the lessons and solutions to problem I faced:

More references:
What’s a GPT?
Booting from GPT
Partitioning Advice
Windows 8 removes Grub as default boot manager
A gdisk Walkthrough