|"Can you run a gaming PC completely on an SSD"|
Yes, but why would you want to? Large capacity SSDs are very expensive. A 512GB SSD costs over $300. You can buy an entire entry level computer at Walmart for under $300!
You'd be better off using a smaller SSD (120-128GB) for the OS & a standard HDD (1TB) to run the programs & store your data.
And STOP asking about RAM!