What should you do if you called the boy you like gay?

It would depend on if you like him or not.

If you do, then tell him. If you don't, then treat him with the same respect as you would any other person (male or female) who confessed to liking you, and let them down gently. An example could be saying "I'm really flattered, but I don't fell the same way".

The main thing, when anyone confesses attraction to you, is to not be rude, threatening or violent toward them.