I currently have a ryzen 5 2400g and rtx 2060 would the cpu load go down if I buy a ryzen 7 3700x?
And by the way, could I then play games like gta and fortnite would play at 120 fps, everything would be high in gta and dchattrn off in fortnite and everything else medium
As long as your graphics card can do it, you would definitely have more fps with the 3700X, but I can't say whether 120fps are always possible.
So the CPU load would definitely go down, since the 3700X is much more powerful. Whether it would be enough for 120 fps depends on the graphics card and the games. I don't know the games you mentioned well enough for that.
However, I wonder why you want to buy it now, 1.5 years after the 3700X was released. It has hardly become cheaper, at the moment hardware is scarce and expensive anyway, and the successor model is already on the market, albeit overpriced and poorly available due to its scarcity.
nvidia and AMD have announced that the delivery situation will relax by the end of January, which I think is credible. I'd rather wait until then and then get a Ryzen 5000.