Either something has messed with your cert store or Oracle is missing a cert that they haven't logged removing.
I recommend getting Java from AdoptOpenJDKL
You don't have the project registered as a Gradle project.
Go to Properties -> Project Natures -> Add -> Gradle Project Nature -> Apply and Close
Then Right Click -> Gradle -> Refresh Gradle Project
They need to not have the slightest possibility that Microsoft could decide they fucked up and shut down Forge.
Right now it sounds as if they're trying to make it sound friendly while leaving a loophole that will allow them to cover their asses at the expense of anyone trying to use the mappings.
Edit: For clarification, when I say "they" I mean MS's lawyers, not Mojang's Dev team
Under the build tab there's likely two columns, in the left column there should be a list of tasks, click the top one and the full log should appear in the right column
The server is always free to download, the difference is whether it checks with the auth servers before accepting players.
Also that's not your issue, it was just a concern I had.
Your actual issue isn't an issue, you're not supposed to be able to connect to your own external IP