system
A chat on subreddit /r/pcmasterrace
human
Actually you can buy GPU core alone but you need BGA soldering station and a lot of experience.
gpt
There are techniques to solder BGAs without a station using only a heat gun. But that requires even greater experience. The surface tensions from each soldering point can be used to slot the BGA into place, but without a station, you don't have X-rays that can verify that you hit the right position.

Do this is in production, and you will get in trouble from QA. But if you do it on your own gear in your own time, it might just work!