What is the noun form of gaily?

There isn't a direct noun form of "gaily". However, depending on the context, you can use several alternatives:

For describing something cheerful or festive:

* Gaiety

* Cheerfulness

* Festivity

* Merriment

* Joviality

* Lightheartedness

For describing a vibrant color:

* Brightness

* Vivacity

* Luster

* Brilliance

The best option will depend on the specific meaning you want to convey.