You were supposed to use gist.github.com which is a separate service but this works.
You're running the cake installation and not the furnace installation, right?
They only appear to yourself and other people using Optifine, they are not registered with Mojang, and selling "capes" is against Minecraft's Commercial EULA.
https://help.mojang.com/customer/en/portal/articles/1590522-minecraft-commercial-use
The old launcher called them profiles, the new launcher calls them Installations, just click the Installations tab, click "New", fill out your options, save, and click play
You must use Java 8, 9, or 10. 11 can't be supported due to a core library breaking compat with Java 8 (the version Mojang ships with the game) when it added support for Java 11
Forge installs itself as a version of Minecraft.
Make a new Profile/Installation and select Forge from the version dropdown.
(Note: For some reason Forge 1.12.2 gets booted to the bottom of the list.)
The filenames of one or more of your mods suggests you got them from a repost site.
Please read up on the StopModReposts project (link in my signature) for info on why this is a bad thing.
Pixelmon requires a specific version of Forge to work correctly,
unfortunately they do not indicate this in their mod file so Forge is unaware and can't show an error screen,
please report this to the Pixelmon Devs.