1- If you know that it is animal leather, and it was purchased from a non-Muslim company/market, and the leather is not from a cold blooded animal (like a snake or lizzard), then yes it is najes. Otherwise it’s not najes.
2- When the leather is najes, if you touch it with wet hands, your hands become najes. If your wet hair touches it, your hair becomes najes.
3- Now it doesn’t mean your ghusl is invalid and has to be redone. The ghusl itself is valid. But when you pray, you have to wash the area if your body that touched the sofa while wet.
4- If you did not know leather is Najes, and it’s not out of negligence that you didn’t know, meaning you made the effort to learn what is najes and what is not, but you simply never came to know that leather is najes, then in this case your previous prayers are all valid. But going forward, make sure if you touch the leather with wet hands, wash your hands before praying.