Possible problems:
Problem with the gpu
To cheap gpu from Ali Express XD
Single fan gpu
Cheap case with bad airflow
Not enough case fans
Possible fixes:
Check if there is a problem with the gpu
Buy fans if you don't use all of the fan slots in your case
If all of this doesn't work, an aftermarket external gpu cooler can help if you have enough space, or some custom water cooling
If the issue is your case, buy a new one (please do this carefully l, cause you might run into compatibility issues, and you will have to rebuild some stuff)