Maximum Operating Temperature of the Ryzen 5 3400g is 95C.
So if it goes up to 80C during heavy loads you are within the processor's Operating Temperature. If you are using a Stock CPU Cooler (AMD CPU Cooler) that probably is normal for heavy game playing.
If it goes up to 90C then you either need to get a more powerful CPU Cooler or make sure your CPU Cooler is working and installed correctly.