Keep in mind that the translation here is wrong and someone will need to fix it.
Basically, if there's a specific "it" that you can point to, use gxi. If not, leave it out.
Gxi estas publika leter - it is a public letter. Gxi estas bona ideo - it is a good idea
There are three cars there. - Estas tri auxtoj tie. It is a warm day - Estas varma tago
Note that if "it" is a placeholder in English it would be left off in Esperanto Estas bona ideo, ke vi faru tion - it's a good idea for you to do that.