What is GMPulse?
GMPulse is a GameMaker extension that makes it possible to manipulate information about your game while it's running. This includes things like monitoring and changing variable values, changing rooms, toggling fullscreen, and more. Being able to change the value of instance or global variables unlocks speedy prototyping, since you no longer need to restart your game every time you want to change something.
How does it work?
GMPulse does not use any native extension business. Instead, it leverages native GML to manipulate everything about your game. The interface for GMPulse is website that is served locally using built-in networking functions. This all means that GMPulse should be compatible with any export that supports networking. (This does have some implications, see Regarding Networking for more info)
How does this compare to YellowAfterLife's GMLive?
GMLive is a powerful live coding solution that enables you to solve complex prototyping concerns by reloading code while your game runs. This makes it a fantastic tool for debugging code, prototyping new pieces of game logic, and reloading game assets. GMLive comes packaged with an external utility that needs to run separate from your game to enable live reloading functionality. A small bit of GML is also required to enable live coding.
GMPulse enables rapid prototyping by making it possible to change the value of any variable in your game while it runs. It makes it easy to quickly experimenting with UI layouts, instance properties, and anything else a variable might control. GMPulse is a zero-setup extension that is fully functional just by adding it to your project. It has a locally hosted web-based GUI, which means you have to have a browser window open (on your computer or another device on your network) to use GMPulse.
GMLive in many cases serves a different purpose than GMPulse, and can do most of the things that GMPulse can, albeit in a different way and with a bit more setup. I wouldn't exactly call them competitors, but rather separate tools that could even be used together in a complimentary manner. If you are looking to add live coding and asset reloading to your project, I would highly suggest grabbing GMLive.