Question Good Gpu for Ryzen 5 2400G?

There is no one right answer to that question. It depends on your budget, your desired frame rates, resolution, and graphics settings, and on what games you play.

With an older "budget" CPU, it probably doesn't make sense to spend a lot of money on a high-end GPU.
 
It depends on your budget, use case and screen resolution.

If you are running Machine Learning workloads, 4K gaming, the only limitation is the PSU in this build. An RTX 5080 would be fitting.

However, if you run games that may face CPU load, an RTX 5080 may be bottlenecked.
On 1440p an RTX 5070/9070 XT would be fine,
on 1080p an RTX5060 Ti or 6800 XT is optimal.
 
I usually add something along the lines of a used RX 580 or GTX 1060 6GB when I put together a build for resale that has an APU like that but needs a bit more power in the graphics department. Since it's going to be for (I presume) a youngster, he'll probably be gaming at 1080p.