what exactly is it and does it do and how to use please i have seen it in a few macros ,thank you in advance
When a GSE Macro executes it has a main body, keypress, keyrelease and pre and post macros. Ignoring Pre and Post macros as they act like the main body in this case. When you press a GSE macro what it prepares to executes is Key Press + the line from main body + Key Release. It will attempt all of those at once. The problem is that there can be only one action in that group that is on the GCD (Global Cooldown). These GCD abilities should be in KeyPress with Non GCD in KeyPress and KeyRelease.
You can put GCD abilities in KeyRelease for extra chances that those abilities will fire when everything else is on cooldown etc.