Kernal panic GPU is not found. PCI config access fails!!!

Honestly... I'd start shopping for a new Mac. This sounds like an impending hardware failure. It's only a matter of time. That machine is not worth repairing.

The simple solution is to not sleep the iMac. I have a 2017 iMac and have never had a problem - because I don't sleep the iMac. I use the screensaver and have it put the monitor to sleep after 10 minutes. If I'm away for any length of time I shut down the iMac as well as at each night.

If the not sleeping doesn't help then I'd take KiltedTim's suggestion seriously.

Here's some food for thought if you're thinking of a new Mac: a 10 Core Mac Mini M4 with 16 GB RAM and 1 TB SSD ($999 + tax) with a 27" LG 4k monitor ($250 + tax from Amazon.com)  is $1249 + tax which is $560 less than a similarly configured new 24" iMac ($1799 + tax)  re 07/2025.   

If your Mac isn't capable of running anything later than Ventura, then it will be obsolete this fall when macOS 26 is released. It will no longer be supported.

You will rapidly lose the ability to make secure connections to web sites, especially those for banking, retail, etc.

It's not worth putting any time or money into an 8 year old Mac that will no longer be able to run a supported version of macOS.

You don't understand... It will soon not be usable for things like surfing. It is going to stop getting updates.

If you want to sink good money into a dying platform, go right ahead. You'd be much better served moving on to something that will be supported, though.

It's a great little machine. The M4 is a real powerhouse. It easily blows any Intel based Mac out of the water.

Note that your old iMac can not be used as a monitor. Not that you were thinking that, but we do get quite a few questions here from people who buy a new Mac thinking they can use their old one as a monitor and same on having to buy one. You can't.

There are plenty of good inexpensive monitors out there in just about any size you might want.