Congratulations
“Gz” is a super short way to say congratulations. It’s actually an abbreviation of an abbreviation – it’s derived from the short form “gratz”, which is itself derived from “congratulations”.
This term is mostly used by teenagers, especially when they are texting or chatting online. They use it to convey sincere congratulations for a significant achievement. However, they may also use it sarcastically to ‘congratulate’ someone on a mishap or misstep.
For instance, you might use “gz” when texting your buddy who just aced a hard exam. Or if your friend shares a picture of his brand new car, you could comment, “It’s awesome, gz!”