HP Prodesk G4 400 MT gaming upgrade

I have an old HP prodesk I bought way back when it first came out. It's been lying idle for some time now. It had an i3 7th-gen with 4 GB of RAM and no dedicated GPU. I don't really play many games, All I need it is to run Fortnite and CS at 1080p 120fps. I'm fine with even 60fps. I don't think the PSU has enough juice to power a GPU with additional power requirements